Business combination, contingent consideration, liability expected holding period.

No definition available.

Name:	alit_BusinessCombinationContingentConsiderationLiabilityExpectedHoldingPeriod
Namespace Prefix:	alit_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Disposal Group, Including Discontinued Operation, Contingent Consideration, Aggregate Principal Amount

+ References

No definition available.

+ Details

Name:	alit_DisposalGroupIncludingDiscontinuedOperationContingentConsiderationAggregatePrincipalAmount
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Disposal Group, Including Discontinued Operation, Contingent Consideration, Aggregate Principal Amount, Fair Value

+ References

No definition available.

+ Details

Name:	alit_DisposalGroupIncludingDiscontinuedOperationContingentConsiderationAggregatePrincipalAmountFairValue
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Equity Instruments, Contingent Consideration Liability, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	alit_EquityInstrumentsContingentConsiderationLiabilityFairValueDisclosure
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The seller earnout fair value of shares that would be issued, determined under the conditions specified in the contract if the settlement were to occur at the reporting date.

+ References

No definition available.

+ Details

Name:	alit_FinancialInstrumentsSubjectToMandatoryRedemptionSettlementTermsSellerEarnOutFairValueOfShares
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Gain (loss) from change in fair value of financial instruments.

+ References

No definition available.

+ Details

Name:	alit_GainLossFromChangeInFairValueOfFinancialInstruments
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Loss (gain) from change in fair value of seller earnouts.

+ References

No definition available.

+ Details

Name:	alit_LossGainFromChangeInFairValueOfSellerEarnouts
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Seller Earnouts, Period To Receive Earn Out Shares

+ References

No definition available.

+ Details

Name:	alit_SellerEarnoutsPeriodToReceiveEarnOutShares
Namespace Prefix:	alit_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Value of input used to measure contingent consideration liability from business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationLiabilityMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Per share or per unit amount of equity securities issued.

+ References

No definition available.

+ Details

Name:	us-gaap_SharesIssuedPricePerShare
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of new stock issued during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-SubTopic 10
-Topic 505
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481004/946-505-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(4)(b))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483575/946-220-S99-3

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(i)(1))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479886/946-10-S99-3

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480008/505-10-S99-1

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=us-gaap_DiscontinuedOperationsDisposedOfBySaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=alit_StradaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=alit_CommonClassZAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=alit_CommonClassZMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=alit_SellerEarnoutsLiabilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputPriceVolatilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputRiskFreeInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments - Additional Information (Details) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 02, 2024 $ / shares shares	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Jul. 12, 2024 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Period to receive earnout shares				7 years
Fair value of seller earnouts				$ 62		$ 95
(Gain) loss from change in fair value of seller earnouts		$ (3)	$ (37)	(32)	$ (24)
(Gain) loss from change in fair value of financial instruments		(23)	(36)	(54)	(11)
Discontinued Operations, Disposed of by Sale \| Strada
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Aggregate principal amount, contingent consideration							$ 150
Fair value contingent consideration		72		72			$ 52
(Gain) loss from change in fair value of financial instruments		20		20
Class A Common Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Shares issued \| shares	1,500,000
Price per share \| $ / shares	$ 7.09
Class Z-A Common Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity, contingent consideration at fair value						$ 13
Class Z Common Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
(Gain) loss from change in fair value of seller earnouts		$ 0	$ 1	$ (2)	$ 13
Seller Earnouts liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Business combination, contingent consideration, liability expected holding period				3 years 9 months 3 days
Seller Earnouts liability \| Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input		0.45		0.45
Seller Earnouts liability \| Risk-free Interest Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement Input		0.0358		0.0358