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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 |  |  |  |  |  |  |  |  |  |

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