Junior Subordinated Debentures (Summary of Junior Subordinated Debentures) (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Subordinated Borrowing [Line Items]
Junior Subordinated Debentures	$ 253,566,000	$ 253,566,000	$ 253,566,000
Junior Subordinated Debt
Subordinated Borrowing [Line Items]
Junior Subordinated Debentures	$ 253,566,000
Debt, weighted average interest rate (as a percent)	4.53%
Junior Subordinated Debt | Wintrust Capital Trust III
Subordinated Borrowing [Line Items]
Common Securities	$ 774,000
Trust Preferred Securities	25,000,000
Junior Subordinated Debentures	$ 25,774,000
Contractual rate (as a percent)	5.60%
Junior Subordinated Debt | Wintrust Capital Trust III | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	3.25%
Junior Subordinated Debt | Wintrust Statutory Trust IV
Subordinated Borrowing [Line Items]
Common Securities	$ 619,000
Trust Preferred Securities	20,000,000
Junior Subordinated Debentures	$ 20,619,000
Contractual rate (as a percent)	5.11%
Junior Subordinated Debt | Wintrust Statutory Trust IV | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	2.80%
Junior Subordinated Debt | Wintrust Statutory Trust V
Subordinated Borrowing [Line Items]
Common Securities	$ 1,238,000
Trust Preferred Securities	40,000,000
Junior Subordinated Debentures	$ 41,238,000
Contractual rate (as a percent)	4.94%
Junior Subordinated Debt | Wintrust Statutory Trust V | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	2.60%
Junior Subordinated Debt | Wintrust Capital Trust VII
Subordinated Borrowing [Line Items]
Common Securities	$ 1,550,000
Trust Preferred Securities	50,000,000
Junior Subordinated Debentures	$ 51,550,000
Contractual rate (as a percent)	4.29%
Junior Subordinated Debt | Wintrust Capital Trust VII | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	1.95%
Junior Subordinated Debt | Wintrust Capital Trust VIII
Subordinated Borrowing [Line Items]
Common Securities	$ 1,238,000
Trust Preferred Securities	25,000,000
Junior Subordinated Debentures	$ 26,238,000
Contractual rate (as a percent)	3.79%
Junior Subordinated Debt | Wintrust Capital Trust VIII | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	1.45%
Junior Subordinated Debt | Wintrust Capital Trust IX
Subordinated Borrowing [Line Items]
Common Securities	$ 1,547,000
Trust Preferred Securities	50,000,000
Junior Subordinated Debentures	$ 51,547,000
Contractual rate (as a percent)	3.97%
Junior Subordinated Debt | Wintrust Capital Trust IX | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	1.63%
Junior Subordinated Debt | Northview Capital Trust I
Subordinated Borrowing [Line Items]
Common Securities	$ 186,000
Trust Preferred Securities	6,000,000
Junior Subordinated Debentures	$ 6,186,000
Contractual rate (as a percent)	5.36%
Junior Subordinated Debt | Northview Capital Trust I | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	3.00%
Junior Subordinated Debt | Town Bankshares Capital Trust I
Subordinated Borrowing [Line Items]
Common Securities	$ 186,000
Trust Preferred Securities	6,000,000
Junior Subordinated Debentures	$ 6,186,000
Contractual rate (as a percent)	5.36%
Junior Subordinated Debt | Town Bankshares Capital Trust I | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	3.00%
Junior Subordinated Debt | First Northwest Capital Trust I
Subordinated Borrowing [Line Items]
Common Securities	$ 155,000
Trust Preferred Securities	5,000,000
Junior Subordinated Debentures	$ 5,155,000
Contractual rate (as a percent)	5.34%
Junior Subordinated Debt | First Northwest Capital Trust I | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	3.00%
Junior Subordinated Debt | Suburban Illinois Capital Trust II
Subordinated Borrowing [Line Items]
Common Securities	$ 464,000
Trust Preferred Securities	15,000,000
Junior Subordinated Debentures	$ 15,464,000
Contractual rate (as a percent)	4.09%
Junior Subordinated Debt | Suburban Illinois Capital Trust II | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	1.75%
Junior Subordinated Debt | Community Financial Shares Statutory Trust II
Subordinated Borrowing [Line Items]
Common Securities	$ 109,000
Trust Preferred Securities	3,500,000
Junior Subordinated Debentures	$ 3,609,000
Contractual rate (as a percent)	3.96%
Junior Subordinated Debt | Community Financial Shares Statutory Trust II | London Interbank Offered Rate (LIBOR)
Subordinated Borrowing [Line Items]
Rate Structure, incremental interest rate over base rate (as a percent)	1.62%