Income Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Revaluation of deferred tax assets			$ 8,721	$ (23,709)	$ 0
Acquired thrift's tax bad debt reserve	$ 2,500		5,800	2,500	2,500
Deferred tax assets, net	64,736		60,129	64,736
Unrecognized tax benefit that would affect effective tax rate	12,900		13,100	12,900	$ 1,400
Northern States Financial Corporation
Income Taxes [Line Items]
Deferred tax assets, net			17,000
Standard Bancshares and NI Bancshares Acquisitions
Income Taxes [Line Items]
Deferred tax assets, net	41,500			41,500
Federal
Income Taxes [Line Items]
Revaluation of deferred tax assets	$ 26,600		$ 8,700	(26,600)
State and Local Jurisdiction
Income Taxes [Line Items]
Revaluation of deferred tax assets		$ 2,800		$ 2,800