Derivative Financial Instruments - Schedule of Consolidated Statement of Income Related to Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Interest rate swaps and caps \| Income Statement Location [Axis]: us-gaap:TradingGainsLosses
Derivative Instruments, Gain (Loss) [Line Items]
Gains on derivative instruments	$ (10)	$ 92	$ (76)	$ 60
Mortgage banking derivatives \| Income Statement Location [Axis]: wtfc:RevenuesMortgageBanking
Derivative Instruments, Gain (Loss) [Line Items]
Gains on derivative instruments	(4,491)	(584)	(1,953)	2,733
Commodity contracts \| Income Statement Location [Axis]: us-gaap:TradingGainsLosses
Derivative Instruments, Gain (Loss) [Line Items]
Gains on derivative instruments	103	89	44	166
Foreign exchange contracts \| Income Statement Location [Axis]: us-gaap:TradingGainsLosses
Derivative Instruments, Gain (Loss) [Line Items]
Gains on derivative instruments	54	79	196	152
Covered call options \| Income Statement Location [Axis]: wtfc:FeesFromCoveredCallOptions
Derivative Instruments, Gain (Loss) [Line Items]
Gains on derivative instruments	4,585	5,619	14,047	14,689
Derivative contract held as economic hedge on MSRs \| Income Statement Location [Axis]: wtfc:RevenuesMortgageBanking
Derivative Instruments, Gain (Loss) [Line Items]
Gains on derivative instruments	$ (27,671)	$ 265	$ (31,967)	$ 7,697