Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Income Taxes [Line Items]
Acquired thrift's tax bad debt reserve	$ 5,800	$ 2,500	$ 5,800
Deferred tax assets, net	60,129		43,382
Unrecognized tax benefit that would affect effective tax rate	13,100	12,900	13,000
Bridgeview Bancorp, Inc.
Income Taxes [Line Items]
Deferred tax assets, net			$ 32,100
Northern States Financial Corporation
Income Taxes [Line Items]
Deferred tax assets, net	17,000
State and Local Jurisdiction
Income Taxes [Line Items]
Revaluation of deferred tax assets		2,800
Internal Revenue Service (IRS)
Income Taxes [Line Items]
Revaluation of deferred tax assets	$ 8,700	$ 26,600