Acquisitions - Summary of Acquired Non-Impaired Loans as of Acquisition Dates (Details) - USD ($) $ in Thousands		Apr. 30, 2019	May 31, 2018
Oak Park River Forest Bankshares, Inc.
Business Acquisition [Line Items]
Fair value		$ 212,587
Gross contractual amounts receivable		275,553
Estimate of contractual cash flows not expected to be collected	[1]	23,932
Estimate of contractual cash flows expected to be collected		$ 251,621
First Evanston Bancorp, Inc.
Business Acquisition [Line Items]
Fair value			$ 890,986
Gross contractual amounts receivable			1,057,374
Estimate of contractual cash flows not expected to be collected	[1]		36,544
Estimate of contractual cash flows expected to be collected			$ 1,020,830

[1] Includes interest payments not expected to be collected due to loan prepayments as well as principal and interest payments not expected to be collected due to customer default.