Accounting Policies [Abstract] Accounting Policies [Abstract] Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Receivable Type [Axis] Receivable Type [Axis] Accounts Payable and Accrued Liabilities Accounts payable, accrued expenses, and other liabilities Accrued Employee Benefits Accrued payables, awards Accrued incentive fees payable. Accrued Incentive Fees Payable Accrued incentive fees payable Accrued management and incentive fees payable. Accrued Management And Incentive Fees Payable Accrued management and incentive fees payable Carrying value as of the balance sheet date of obligations incurred through that date and payable for management fees. Accrued Management Fees Payable Accrued management fees payable Accrued special servicing fee payable. Accrued Special Servicing Fee Payable Accrued special servicing fee payable Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax Gain related to changes in fair value of derivative instruments Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Cumulative currency translation adjustment on assets and liabilities denominated in foreign currencies Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated other comprehensive loss Accumulated other comprehensive loss AOCI Attributable to Parent [Member] Accumulated Other Comprehensive Income (loss) [Member] Activity In Loans Receivable Table [Text Block] Activity In Loans Receivable Table [Text Block] Activity Relating to Loans Receivable Portfolio Additional Cash and Cash Equivalent Related Text Cash and cash equivalents, description Additional Paid in Capital, Common Stock Additional paid-in capital Additional Paid-in Capital [Member] Additional Paid-In Capital [Member] Additions During Period [Abstract] Additions During Period [Abstract] Additions during period: The increase (decrease) during the reporting period in the obligation related to deferred compensation for Independent (non-executive) Board of Directors' members. Adjustments To Additional Paid In Capital Deferred Compensation Deferred directors' compensation Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments Issuance of convertible notes Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options Restricted class A common stock earned Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to net cash provided by operating activities Adjustment To Par Value For Reverse Stock Split Adjustment To Par Value For Reverse Stock Split Adjustment to par value for reverse stock split and charter amendment Administrative Fees Expense Administrative services expenses incurred An affiliate is a party that, directly or indirectly through one or more intermediaries, controls, is controlled by, or is under common control with the entity. Affiliated Entity 2 [Member] Affiliate of our Manager [Member] Affiliated Entity [Member] Affiliates of Manager [Member] Aggregate sales price allowed under At the Market Agreement. Aggregate Sales Price of Common Stock Aggregate sales price Aggregate sales price of common stock remaining available. Aggregate Sales Price of Common Stock Remaining Available Aggregate sales price remaining available Agreement Initiation Date Agreement Initiation Date Joint venture agreement date All Currencies [Domain] All Currencies [Domain] Allocated Share-based Compensation Expense Director stock-based compensation Allowance for Loan and Lease Losses, Write-offs Charge-offs Amendment Flag Amendment Flag Amortization Of Debt Discount And Debt Issuance Costs Amortization Of Debt Discount And Debt Issuance Costs Interest on convertible notes related to amortization of discount and certain issuance costs Amortization of deferred fees and other items on loans. Amortization Of Deferred Fees And Other Items On Loans Amortization of deferred fees and other items on loans Amortization of deferred financing costs and other items on debt obligations. Amortization Of Deferred Financing Costs And Other Items On Debt Obligations Amortization of deferred financing costs and other items on debt obligations Amortization of Deferred Loan Origination Fees, Net Amortization of deferred fees and expenses Amortization of fees and other items Amount of guarantee obligations. Amount of Guarantee Obligations Guarantee of outstanding borrowings The percentage of the required distribution for such calendar year. Annual Distribution Of Net Taxable Income Percentage Annual distribution of net taxable income for U.S. federal income tax not to apply to our earnings that we distribute (percent) Annual Limitation Net Operating Losses And Net Capital Losses Annual Limitation Net Operating Losses And Net Capital Losses Net operating losses and net capital losses April 2015 Stock Offering [Member] April 2015 Stock Offering [Member] April 2015 Stock Offering [Member] Assets Total Assets Assets [Abstract] Assets Assets, Fair Value Disclosure [Abstract] Assets Asset Specific Borrowings [Member] Asset Specific Borrowings [Member] Asset Specific Borrowings [Member] Asset Specific Repurchase Agreements [Member] Asset Specific Repurchase Agreements [Member] Asset-Specific Repurchase Agreements [Member] Asset Specific Repurchase Agreements Number Of Loans Asset Specific Repurchase Agreements Number Of Loans Number of loans Asset Specific Revolving Repurchase Facilities [Member] Asset Specific Revolving Repurchase Facilities [Member] Asset Specific Revolving Repurchase Facilities [Member] At The Market Agreement [Member] At The Market Agreement [Member] ATM Agreements [Member] Audit Director [Member] Audit Director [Member] Chairperson of Audit Committee [Member] Award Type [Axis] Award Type [Axis] Identification of the name of counterparty to the derivative instrument. Bank Of America [Member] Bank of America [Member] Base Rate [Member] Base Rate [Member] Basis of Accounting, Policy [Policy Text Block] Basis of Presentation Basis of Presentation and Significant Accounting Policies [Text Block] Summary of Significant Accounting Policies The board of directors (who collectively have responsibility for determining the overall policy of the entity and appointing officers) generally elected by the shareholders. Board Of Directors [Member] Five Independent Board of Directors [Member] Book value collateral of gross loan for asset specific repurchase agreements. Book Value Collateral Of Gross Loan For Asset Specific Repurchase Agreements Book value Book Value Net [Member] Book Value Net [Member] Net Book Value [Member] Borrower A [Member] Borrower A [Member] Borrower A [Member] Borrower B [Member] Borrower B [Member] Borrower B [Member] Borrower C [Member] Borrower C [Member] Borrower C [Member] Borrower D [Member] Borrower D [Member] Borrower D [Member] Borrower E [Member] Borrower E [Member] Borrower E [Member] Bxmt Advisors Limited Liability Company And Affiliates [Member] Bxmt Advisors Limited Liability Company And Affiliates [Member] BXMT Advisors Limited Liability Company and Affiliates [Member] BXMT Advisors Limited Liability Company [Member] BXMT Advisors Limited Liability Company [Member] Manager [Member] CANADA Canada [Member] Canada, Dollars CAD [Member] Reported Value Measurement [Member] Carrying Amount [Member] Carrying value of portfolio loans restructured. Carrying Value Of Portfolio Loans Restructured Carrying value of loans Cash and Cash Equivalents, at Carrying Value Cash and cash equivalents at end of year Cash and cash equivalents at beginning of year Cash and cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Net increase in cash and cash equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block] Restricted Cash Cash Flow Hedging [Member] Cash Flow Hedges [Member] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Supplemental disclosure of non-cash investing and financing activities Cdor [Member] Cdor [Member] CDOR [Member] Citibank [Member] Citibank [Member] Citibank [Member] Class of Stock [Domain] Class of Stock [Domain] Class of Stock [Line Items] Class of Stock [Line Items] Closing Of Asset Specific Repurchase Facilities Closing Of Asset Specific Repurchase Facilities Closing of asset specific repurchase facilities Closing Of Revolving Repurchase Facility Closing Of Revolving Repurchase Facility Number of revolving repurchase facilities closed Collateral Assets [Member] Collateral Assets [Member] Collateral Assets [Member] Collateral [Axis] Collateral [Axis] Collateral [Domain] Collateral [Domain] Commercial Mortgage Backed Securities [Member] Commercial Mortgage Backed Securities [Member] Commercial Portfolio Segment [Member] Commercial Portfolio Segment [Member] Commitments and Contingencies Commitments and contingencies Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Text Block] Commitments and Contingencies Commitments And Contingencies [Line Items] Commitments And Contingencies [Line Items] Commitments And Contingencies [Line Items] Commitments And Contingencies [Table] Commitments And Contingencies [Table] Commitments And Contingencies [Table] Period (in number of years) where the unfunded commitments (future commitments not funded at time of closing) related to our outstanding loans will expire. Commitments Expiration Period Commitments expiration period Common Class A [Member] Class A Common Stock [Member] Common Stock, Capital Shares Reserved for Future Issuance Reserved for issuance of class A common stock Common Stock, Dividends, Per Share, Cash Paid Dividends paid per common stock Common Stock, Dividends, Per Share, Declared Dividends declared per share of common stock Common Stock [Member] Common Stock [Member] Common Stock, Par or Stated Value Per Share Common stock, par value Common stock, par value in dollars per share Common Stock, Shares Authorized Common stock, shares authorized Common Stock, Shares, Issued Common stock, shares issued Common Stock, Shares, Outstanding Common stock, shares outstanding Common Stock Shares Outstanding Including Stock Equivalents Common Stock Shares Outstanding Including Stock Equivalents Ending balance Beginning balance Common Stock Shares Outstanding Percentage Common Stock Shares Outstanding Percentage Common stock, shares outstanding percentage Common Stock, Value, Issued Class A common stock, $0.01 par value, 200,000,000 shares authorized, 93,702,326 and 58,269,889 shares issued and outstanding as of December 31, 2015 and 2014, respectively Common stock value Comprehensive Income (Loss), Net of Tax, Attributable to Parent Comprehensive income attributable to Blackstone Mortgage Trust, Inc. Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive income attributable to non-controlling interests Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Comprehensive income The percentage per annum multiplied by outstanding equity balance in computation of management fees. Computed Management Fee Percent Of Average Outstanding Equity Balance Management fee - percent of outstanding equity balance Condo [Member] Condo [Member] Condo / Northeast [Member] Condominium Property [Member] Condominium Property [Member] Condominium [Member] Consolidation, Policy [Policy Text Block] Principles of Consolidation Conversion of Stock, Description Description of Convertible Notes conversion Convertible Debt, Fair Value Disclosures Convertible notes, net Convertible Debt [Member] Convertible Notes [Member] Convertible Notes Payable Convertible notes Convertible notes, net Convertible Notes Payable [Member] Convertible Senior Notes [Member] Convertible Notes Policy [Text Block] Convertible Notes Policy [Text Block] Convertible Notes Convertible Notes [Text Block] Convertible Notes [Text Block] Convertible Notes, Net Core, general and administrative expenses. Core General And Administrative Expenses Subtotal Counterparty Name [Axis] Counterparty Name [Axis] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] CT CDO I [Member] Ct Cdo I [Member] CT CDO I [Member] CT Investment Management Co LLC [Member] CT Investment Management Co LLC [Member] CT Investment Management Co LLC [Member] CT Legacy Partners [Member] CT Legacy Partners [Member] CT Legacy Partners [Member] CTOPI [Member] C T O P I [Member] CT Opportunity Partners I, LP [Member] Currency [Axis] Currency [Axis] Current Fiscal Year End Date Current Fiscal Year End Date GERMANY Germany [Member] Debt Conversion, Original Debt, Amount Convertible Notes, debt conversion, principal amount Debt Disclosure [Abstract] Debt Disclosure [Abstract] Debt Disclosure [Text Block] Secured Debt Agreements Debt instrument asset specific borrowings advance rate. Debt Instrument Asset Specific Borrowings Advance Rate Weighted average rate Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Basis Spread on Variable Rate Rate Interest Payment Rates LIBOR basis spread on debt obligation, in percent Debt Instrument, Collateral Amount Collateral Assets Debt Instrument, Convertible, Conversion Ratio Convertible Notes, debt conversion, common stock issued, in percent Debt instrument covenant fixed charge coverage ratio. Debt Instrument Covenant Fixed Charge Coverage Ratio1 Covenants, EBITDA to fixed charges, in percent Debt Instrument Covenant Liquidity Debt Instrument Covenant Liquidity Covenants, minimum cash liquidity amount Debt Instrument Covenant, Minimum Percentage Of Cash Liquidity On Recourse Indebtedness Debt Instrument Covenant, Minimum Percentage Of Cash Liquidity On Recourse Indebtedness Covenants, percentage of recourse indebtedness Debt Instrument Covenant, Minimum Percentage Of Tangible Net Worth On Net Cash Proceed Of Equity Issuances Debt Instrument Covenant, Minimum Percentage Of Tangible Net Worth On Net Cash Proceed Of Equity Issuances Covenants, percentage of tangible assets on cash proceeds from equity issuances Debt Instrument Covenant Requirement On Consolidated Tangible Net Worth Minimum Debt Instrument Covenant Requirement On Consolidated Tangible Net Worth Minimum Covenants, minimum tangible net worth Debt Instrument, Description of Variable Rate Basis Debt instrument variable rate basis Debt Instrument Effective Interest Rate Debt Instrument Effective Interest Rate Convertible Notes, assumed effective interest rate Debt Instrument, Face Amount Convertible Senior Notes, Principal amount Debt Instrument, Fair Value Disclosure Secured debt agreements Debt Instrument, Interest Rate, Effective Percentage Convertible Senior Notes, Interest rate including amortization of discount upon issuance Debt Instrument, Interest Rate, Stated Percentage Convertible Senior Notes, Interest rate Fixed Interest Payment Rates Debt Instrument Interest Rate Stated Percentage Including Amortization Cost Debt Instrument Interest Rate Stated Percentage Including Amortization Cost Convertible Senior Notes, Interest rate including underwriter discounts Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum Fixed Interest Payment Rates, Maximum Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum Fixed Interest Payment Rates, Minimum Debt Instrument [Line Items] Debt Instrument [Line Items] Debt Instrument, Maturity Date Maturity date Debt Instrument, Maturity Date, Description Borrowings maturity term Term/Maturity Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Debt Instrument, Number of Extension Options Debt Instrument, Number of Extension Options Number of extension options Debt Instrument Revolving Repurchase Facilities Advance Rate Debt Instrument Revolving Repurchase Facilities Advance Rate Advance Rate Weighted-average advance rate Debt Instrument Revolving Repurchase Facilities Guarantee Rate Debt Instrument Revolving Repurchase Facilities Guarantee Rate Guarantee Debt Instrument Revolving Repurchase Facilities Margin Call Description Debt Instrument Revolving Repurchase Facilities Margin Call Description Margin Call Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument, Term Term of maturity date Debt Instrument, Unamortized Discount Discount upon issuance of Convertible Notes Debt Issuance Cost Debt issuance costs Debt, Policy [Policy Text Block] Deferred Financing Costs Deconsolidation, Gain (Loss), Amount Loss on deconsolidation of subsidiary Loss on deconsolidation Loss on deconsolidation of subsidiary Deductions During Period [Abstract] Deductions During Period [Abstract] Deductions during period: Deferred Bonus and Profit Sharing Arrangement, Individual Contract, Type of Deferred Compensation [Domain] Deferred Bonus and Profit Sharing Arrangement, Individual Contract, Type of Deferred Compensation [Domain] Deferred Compensation Arrangement with Individual, Compensation Expense Accrued expenses for incentive plan Deferred Compensation Arrangement with Individual, Description Description of incentive management fee awards to employees vesting schedule Description of incentive management fee grants to employees vesting schedule Deferred Bonus and Profit Sharing Arrangements, Individual Contracts, Type of Deferred Compensation [Axis] Deferred Bonus and Profit Sharing Arrangements, Individual Contracts, Type of Deferred Compensation [Axis] Deferred Compensation Arrangement with Individual, Shares Issued Issuance of deferred stock units Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] Deferred Fees And Other Items [Member] Deferred Fees And Other Items [Member] Deferred Fees/Other Items [Member] Primarily relates to purchase discounts or premiums, exit fees, and deferred origination expenses. Represents the difference between principal balance and net book value of the loan portfolio. Deferred Fees and Other Items on Loan Principal Balance Deferred fees and other items Deferred Finance Costs, Gross Unamortized deferred financing costs Deferred Finance Costs, Net Deferred financing costs Deferred financing costs Deferred Income Allocation Deferred Income Allocation Deferred income allocation Deferred Tax Liabilities, Gross Deferred tax liabilities Deferred Origination Fees and Expenses Deferred Origination Fees and Expenses Deferred origination fees and expenses Deferred Profit Sharing [Member] CTOPI Incentive Plan [Member] Deferred Revenue, Description Income accrual, description Deferred Stock Units Deferred Stock Units Deferred stock units held by directors Deferred Tax Assets, Gross Deferred tax assets Deferred Tax Liabilities, Deferred Expense, Deferred Financing Costs Deferred financing costs Derivative Asset, Fair Value, Gross Liability Fair Value of Derivatives in a Liability Position Derivative Asset Derivatives Derivative, Average Remaining Maturity Wtd. Avg. Maturity (Years) Derivative, by Nature [Axis] Derivative, by Nature [Axis] Derivative, Cap Interest Rate Strike Derivative, Cash Received on Hedge Net cash settlements on our foreign currency forward contracts Derivative contract interest rate swap termination period. Derivative Contract Interest Rate Swap Termination Period Interest rate swaps, termination period Derivative Contract [Domain] Derivative Contract [Domain] Derivative Asset, Fair Value, Gross Asset Fair Value of Derivatives in an Asset Position Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosures [Line Items] Derivative Instruments and Hedging Activities Disclosures [Line Items]