Junior Subordinated Debentures (Summary of Junior Subordinated Debentures) (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Subordinated Borrowing [Line Items]
Junior Subordinated Debentures	$ 253,566	$ 253,566	$ 253,566
Junior Subordinated Debt
Subordinated Borrowing [Line Items]
Junior Subordinated Debentures	$ 253,566	$ 253,600	$ 253,600
Average contractual rate	2.32%
Junior Subordinated Debt | Wintrust Capital Trust III
Subordinated Borrowing [Line Items]
Common Securities	$ 774
Trust  Preferred Securities	25,000
Junior Subordinated Debentures	$ 25,774
Contractual rate	3.38%
Junior Subordinated Debt | Wintrust Capital Trust III | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	3.25%
Junior Subordinated Debt | Wintrust Statutory Trust IV
Subordinated Borrowing [Line Items]
Common Securities	$ 619
Trust  Preferred Securities	20,000
Junior Subordinated Debentures	$ 20,619
Contractual rate	2.93%
Junior Subordinated Debt | Wintrust Statutory Trust IV | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	2.80%
Junior Subordinated Debt | Wintrust Statutory Trust V
Subordinated Borrowing [Line Items]
Common Securities	$ 1,238
Trust  Preferred Securities	40,000
Junior Subordinated Debentures	$ 41,238
Contractual rate	2.73%
Junior Subordinated Debt | Wintrust Statutory Trust V | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	2.60%
Junior Subordinated Debt | Wintrust Capital Trust VII
Subordinated Borrowing [Line Items]
Common Securities	$ 1,550
Trust  Preferred Securities	50,000
Junior Subordinated Debentures	$ 51,550
Contractual rate	2.07%
Junior Subordinated Debt | Wintrust Capital Trust VII | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	1.95%
Junior Subordinated Debt | Wintrust Capital Trust VIII
Subordinated Borrowing [Line Items]
Common Securities	$ 1,238
Trust  Preferred Securities	25,000
Junior Subordinated Debentures	$ 26,238
Contractual rate	1.58%
Junior Subordinated Debt | Wintrust Capital Trust VIII | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	1.45%
Junior Subordinated Debt | Wintrust Capital Trust IX
Subordinated Borrowing [Line Items]
Common Securities	$ 1,547
Trust  Preferred Securities	50,000
Junior Subordinated Debentures	$ 51,547
Contractual rate	1.75%
Junior Subordinated Debt | Wintrust Capital Trust IX | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	1.63%
Junior Subordinated Debt | Northview Capital Trust I
Subordinated Borrowing [Line Items]
Common Securities	$ 186
Trust  Preferred Securities	6,000
Junior Subordinated Debentures	$ 6,186
Contractual rate	3.13%
Junior Subordinated Debt | Northview Capital Trust I | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	3.00%
Junior Subordinated Debt | Town Bankshares Capital Trust I
Subordinated Borrowing [Line Items]
Common Securities	$ 186
Trust  Preferred Securities	6,000
Junior Subordinated Debentures	$ 6,186
Contractual rate	3.13%
Junior Subordinated Debt | Town Bankshares Capital Trust I | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	3.00%
Junior Subordinated Debt | First Northwest Capital Trust I
Subordinated Borrowing [Line Items]
Common Securities	$ 155
Trust  Preferred Securities	5,000
Junior Subordinated Debentures	$ 5,155
Contractual rate	3.13%
Junior Subordinated Debt | First Northwest Capital Trust I | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	3.00%
Junior Subordinated Debt | Suburban Illinois Capital Trust II
Subordinated Borrowing [Line Items]
Common Securities	$ 464
Trust  Preferred Securities	15,000
Junior Subordinated Debentures	$ 15,464
Contractual rate	1.87%
Junior Subordinated Debt | Suburban Illinois Capital Trust II | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	1.75%
Junior Subordinated Debt | Community Financial Shares Statutory Trust II
Subordinated Borrowing [Line Items]
Common Securities	$ 109
Trust  Preferred Securities	3,500
Junior Subordinated Debentures	$ 3,609
Contractual rate	1.74%
Junior Subordinated Debt | Community Financial Shares Statutory Trust II | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure	1.62%