Selected Quarterly Financial Data (unaudited) - Additional Information (Details) - USD ($)	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Line Items]
Net income (loss)	$ (766,000)	$ 9,755,000	$ 6,146,000	$ 6,560,000	$ 63,915,000	$ 1,565,000	$ 2,601,000	$ (1,352,000)			$ 21,695,000	$ 66,729,000	$ (14,974,000)
Basic earnings per common share	$ (0.03)	$ 0.33	$ (0.18)	$ 0.26	$ 2.66	$ 0.08	$ 0.13	$ (0.08)			$ 0.39	$ 3.31	$ (0.86)
Net interest income before provision for loan losses	$ 32,151,000	$ 31,412,000	$ 29,811,000	$ 29,538,000	$ 28,252,000	$ 20,865,000	$ 20,381,000	$ 21,120,000			$ 122,912,000	$ 90,618,000	$ 76,632,000
Incremntal tax expense from enactment of Tax Cuts and Jobs Act of 2017	7,200,000										$ 7,200,000
Corporate federal income tax rate											35.00%	35.00%	35.00%
Income tax expense (benefit)	$ (11,851,000)	$ 1,390,000	(4,094,000)	$ (4,544,000)	61,023,000	$ (9,000)	$ (9,000)	$ 240,000			$ (19,099,000)	$ 61,245,000	$ (307,000)
Deferred tax assets net of valuation allowance					$ 61,900,000							$ 61,900,000
Scenario, Plan
Quarterly Financial Information Disclosure [Line Items]
Corporate federal income tax rate										21.00%
Illinois | State
Quarterly Financial Information Disclosure [Line Items]
Corporate income tax rate		7.00%							5.25%
State income tax benefit due to increased value of deferred tax asset related to net loss deduction		$ 4,600,000
IPO | Series A Preferred Stock
Quarterly Financial Information Disclosure [Line Items]
Dividends in arrears			$ 10,500,000