Loan and Lease Receivables - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Apr. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	May 31, 2018	Dec. 31, 2017
Accounts Notes And Loans Receivable [Line Items]
Total loans and leases	$ 3,826,735			$ 3,499,463
Loans held for sale pledged as security for borrowings	1,900,000			1,400,000
Allowance for loan and lease losses	31,585	$ 31,132		25,201	$ 23,424	$ 19,687		$ 16,706
Acquired Impaired Loans
Accounts Notes And Loans Receivable [Line Items]
Allowance for loan and lease losses	2,800			2,700
Acquired Impaired Loans | First Evanston Bancorp, Inc.
Accounts Notes And Loans Receivable [Line Items]
Loans acquired							$ 25,025
Acquired Impaired Loans | Oak Park River Forest Bankshares, Inc.
Accounts Notes And Loans Receivable [Line Items]
Loans acquired			$ 48,564
Acquired Non-Impaired Loans and Leases | Oak Park River Forest Bankshares, Inc.
Accounts Notes And Loans Receivable [Line Items]
Loans acquired	212,600
Acquired Non-Impaired Loans and Leases | First Evanston
Accounts Notes And Loans Receivable [Line Items]
Loans acquired	891,000
Installment and Other
Accounts Notes And Loans Receivable [Line Items]
Total loans and leases	9,284			13,675
Overdraft deposits reclassified as loans	2,000			1,700
Allowance for loan and lease losses	55	$ 67		49	$ 38	$ 30		$ 41
U.S. Government Guaranteed Loans
Accounts Notes And Loans Receivable [Line Items]
Total loans and leases	$ 139,800			$ 108,700