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Borrowings - Additional information (Details) - USD ($)	3 Months Ended		12 Months Ended
Borrowings - Additional information (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2016	Dec. 31, 2018	Apr. 20, 2018
Borrowings
Interest rate (as a percent)		5.75%
Bank owned FHLBC stock	$ 5,000,000		$ 7,200,000
Senior notes	44,183,000		44,158,000
Debt Instrument, Term		10 years
Interest rate term		5 years
Total borrowings	242,449,000		313,355,000
GCFC/ABC Bank
Borrowings
FHLBC advance amount	$ 13,200,000
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)	1.60%
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	$ 42,400,000		46,600,000
Fair value of the pledged collateral	75,800,000		72,800,000
Total borrowings	$ 42,361,000		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	$ 85,000,000		145,500,000
Bank owned FHLBC stock	5,000,000
Fair value of securities collateralized	78,200,000
Principal balance of loans collateralized	758,700,000
Combined collateral value	$ 577,700,000
Percentage increase in collateral pledged in support of federal home loan bank advances	482.10%
Amount available for additional borrowings	$ 405,100,000
Total borrowings	$ 85,000,000		149,500,000
FHLB advances at 2.54%
Borrowings
Interest rate (as a percent)	2.54%
FHLBC advance amount	$ 70,000,000
FHLB advances at 1.60% to 2.56%
Borrowings
FHLBC advance amount	$ 15,000,000
FHLB advances at 1.60% to 2.56% \| Minimum [Member]
Borrowings
Interest rate (as a percent)	1.60%
FHLB advances at 1.60% to 2.56% \| Maximum [Member]
Borrowings
Interest rate (as a percent)	2.56%
Notes Payable to Banks [Member]
Borrowings
Total borrowings	$ 13,207,000		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	57,698,000		57,686,000
Senior Notes.
Borrowings
Senior notes	44,183,000		44,158,000
Debt issuance costs	$ 817,000		$ 842,000
Amortization period for deferred financing costs	10 years