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Deposits - Narrative (Details) - USD ($) $ in Billions | Dec. 31, 2025 | Dec. 31, 2024
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Deposits [Abstract] |  |
Time deposits of $250,000 or more | $ 4.0 | $ 3.9

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- References No definition available. + Details
Name: | us-gaap_DepositsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Time Deposits, $250,000 Or More + References No definition available. + Details
Name: | wtfc_TimeDeposits250000OrMore
Namespace Prefix: | wtfc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant