Deposits (Narrative) (Details) - USD ($) $ in Billions	Dec. 31, 2018	Dec. 31, 2017
Deposits [Abstract]
Time deposits of $250,000 or more	$ 1.6	$ 1.3

Time Deposits, Two Hundred Fifty Thousand Dollars Or More

No definition available.

Name:	wtfc_TimeDepositsTwoHundredFiftyThousandDollarsOrMore
Namespace Prefix:	wtfc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant