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Borrowings - Additional information (Details) - USD ($)	3 Months Ended	9 Months Ended
	Dec. 31, 2016	Sep. 30, 2018	Jun. 30, 2018	Apr. 20, 2018	Dec. 31, 2017
Borrowings
Interest rate (as a percent)	5.75%
Bank owned FHLBC stock		$ 4,300,000			$ 5,400,000
Senior notes		44,133,000			44,058,000
Debt Instrument, Term	10 years
Interest rate term	5 years
Total borrowings			$ 246,065,000		246,615,000
GCFC/ABC Bank
Borrowings
FHLBC advance amount		$ 18,100,000
Subordinated Debt.				$ 6,600,000
Debt Instrument, Term		7 years 6 months
Debt assumed		$ 23,500,000
GCFC/ABC Bank | Minimum
Borrowings
Interest rate (as a percent)		1.40%
GCFC/ABC Bank | Maximum
Borrowings
Interest rate (as a percent)		2.83%
Three month LIBOR
Borrowings
Basis points added to reference rate (as a percent)	385.00%
Securities sold under repurchase agreements
Borrowings
Carrying amount of securities secured			44,300,000		29,900,000
Fair value of the pledged collateral			72,600,000		40,000,000
Total borrowings			44,333,000		29,918,000
Securities sold under repurchase agreements | Minimum
Borrowings
Threshold percentage of stockholders' equity		10.00%
FHLBC advances
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		$ 77,900,000			115,000,000
Bank owned FHLBC stock			4,300,000
Fair value of securities collateralized		$ 77,300,000
Principal balance of loans collateralized			304,200,000
Combined collateral value			303,300,000
Amount available for additional borrowings			133,000,000
Total borrowings			81,875,000		115,000,000
FHLB advances at 2.01 %
Borrowings
Interest rate (as a percent)		2.22%
FHLBC advance amount		$ 65,000,000
FHLB advances at 1.40 %
Borrowings
Interest rate (as a percent)		2.24%
FHLBC advance amount		$ 5,000,000
Notes payable and other borrowings
Borrowings
Total borrowings			18,050,000
Correspondent Bank
Borrowings
Interest rate (as a percent)		3.85%
Maximum borrowing capacity		$ 20,000,000
Debt Instrument, Term		360 days
Short-term debt		$ 4,000,000
Junior subordinated debentures
Borrowings
Total borrowings			57,674,000		57,639,000
Senior Notes.
Borrowings
Senior notes			44,133,000		44,058,000
Debt issuance costs			$ 867,000		$ 942,000
Amortization period for deferred financing costs		10 years

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Represents the amount of borrowings from Federal Home Loan Bank as a percentage of book value of certain mortgage loans.

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Amount of long-term debt due within one year or within the normal operating cycle, if longer, assumed at the acquisition date.

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