Derivative Financial Instruments - Schedule of Consolidated Statement of Income Related to Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Interest rate swaps and caps \| Income Statement Location [Axis]: us-gaap:TradingGainsLosses
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) on derivative instruments	$ (18)	$ 85	$ (66)	$ (32)
Mortgage banking derivatives \| Income Statement Location [Axis]: wtfc:RevenuesMortgageBanking
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) on derivative instruments	36	(324)	2,538	3,317
Commodity contracts \| Income Statement Location [Axis]: us-gaap:TradingGainsLosses
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) on derivative instruments	(44)	(37)	(59)	77
Foreign exchange contracts \| Income Statement Location [Axis]: us-gaap:TradingGainsLosses
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) on derivative instruments	87	65	142	73
Covered call options \| Income Statement Location [Axis]: wtfc:FeesFromCoveredCallOptions
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) on derivative instruments	4,793	5,624	9,462	9,070
Derivative contract held as economic hedge on MSRs \| Income Statement Location [Axis]: wtfc:RevenuesMortgageBanking
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) on derivative instruments	$ (3,396)	$ 2,535	$ (4,296)	$ 7,432