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Borrowings - Additional information (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2016 | Sep. 30, 2019 | Dec. 31, 2018 | Apr. 20, 2018
Borrowings |  |  |  |
Interest rate (as a percent) | 5.75% |  |  |
Bank owned FHLBC stock |  | $ 4,500,000 | $ 7,200,000 |
Senior notes |  | 44,244,000 | 44,158,000 |
Debt Instrument, Term | 10 years |  |  |
Interest rate term | 5 years |  |  |
Total borrowings |  | 243,692,000 | 313,355,000 |
GCFC/ABC Bank |  |  |  |
Borrowings |  |  |  |
FHLBC advance amount |  | $ 8,900,000 |  |
Subordinated Debt. |  |  |  | $ 6,600,000
Debt Instrument, Term |  | 7 years |  |
Debt assumed |  | $ 23,400,000 |  |
GCFC/ABC Bank | Minimum [Member] |  |  |  |
Borrowings |  |  |  |
Interest rate (as a percent) |  | 2.19% |  |
GCFC/ABC Bank | Maximum [Member] |  |  |  |
Borrowings |  |  |  |
Interest rate (as a percent) |  | 2.83% |  |
Debt Instrument, Variable Rate Base LIBOR [Member] |  |  |  |
Borrowings |  |  |  |
Basis points added to reference rate (as a percent) | 385.00% |  |  |
Securities Sold under Agreements to Repurchase [Member] |  |  |  |
Borrowings |  |  |  |
Carrying amount of securities secured |  | $ 48,900,000 | 46,600,000 |
Fair value of the pledged collateral |  | 62,400,000 | 72,800,000 |
Total borrowings |  | $ 48,870,000 | 46,632,000 |
Securities Sold under Agreements to Repurchase [Member] | Minimum [Member] |  |  |  |
Borrowings |  |  |  |
Threshold percentage of stockholders' equity |  | 10.00% |  |
Federal Home Loan Bank Advances [Member] |  |  |  |
Borrowings |  |  |  |
Borrowings at FHLBC as percentage of total assets |  | 35.00% |  |
Borrowings at FHLBC as percentage of book value of certain mortgage loans |  | 60.00% |  |
FHLBC advance amount |  | $ 84,000,000 | 145,500,000 |
Bank owned FHLBC stock |  | 4,500,000 |  |
Fair value of securities collateralized |  | 54,900,000 |  |
Principal balance of loans collateralized |  | 610,300,000 |  |
Combined collateral value |  | $ 464,100,000 |  |
Percentage increase in collateral pledged in support of federal home loan bank advances |  | 367.40% |  |
Amount available for additional borrowings |  | $ 277,000,000 |  |
Total borrowings |  | $ 84,000,000 | 149,500,000 |
FHLB advances at 2.02% |  |  |  |
Borrowings |  |  |  |
Interest rate (as a percent) |  | 2.02% |  |
FHLBC advance amount |  | $ 65,000,000 |  |
FHLB advances at 2.04% |  |  |  |
Borrowings |  |  |  |
FHLBC advance amount |  | 10,000,000 |  |
FHLB advances at 1.76% to 2.23% |  |  |  |
Borrowings |  |  |  |
FHLBC advance amount |  | $ 8,900,000 |  |
FHLB advances at 1.76% to 2.23% | Minimum [Member] |  |  |  |
Borrowings |  |  |  |
Interest rate (as a percent) |  | 1.76% |  |
FHLB advances at 1.76% to 2.23% | Maximum [Member] |  |  |  |
Borrowings |  |  |  |
Interest rate (as a percent) |  | 2.23% |  |
Notes Payable to Banks [Member] |  |  |  |
Borrowings |  |  |  |
Total borrowings |  | $ 8,856,000 | 15,379,000 |
Credit Facility [Member] |  |  |  |
Borrowings |  |  |  |
Maximum borrowing capacity |  |  | $ 20,000,000 |
Debt Instrument, Term |  |  | 360 days |
Short-term debt |  |  | $ 4,000,000 |
Junior Subordinated Debt [Member] |  |  |  |
Borrowings |  |  |  |
Total borrowings |  | 57,722,000 | 57,686,000 |
Senior Notes. |  |  |  |
Borrowings |  |  |  |
Senior notes |  | 44,244,000 | 44,158,000 |
Debt issuance costs |  | $ 756,000 | $ 842,000 |
Amortization period for deferred financing costs |  | 10 years |  |

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