Borrowings - Additional information (Details)	12 Months Ended
Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Borrowings
Threshold percentage of stockholders' equity	10.00%
Borrowings at FHLBC as percentage of total assets	35.00%
Bank owned FHLBC stock	$ 5,400,000	$ 3,100,000
Fair value of securities collateralized	80,900,000
Principal balance of loans collateralized	277,400,000
Maximum borrowing capacity	286,300,000
Amount available for additional borrowings	132,000,000
Senior notes	44,058,000	43,998,000
Debt issuance costs	739,000	787,000
Total borrowings	246,615,000	197,304,000
Securities sold under repurchase agreements
Borrowings
Carrying amount of securities secured	29,900,000	25,700,000
Fair value of the pledged collateral	$ 40,000,000	43,000,000
Number of customers having secured balances exceeding specified percentage of stockholders equity \| item	0
Total borrowings	$ 29,918,000	$ 25,715,000
Securities sold under repurchase agreements \| Minimum
Borrowings
Maturity	1 day
Securities sold under repurchase agreements \| Maximum
Borrowings
Maturity	90 days
FHLBC advances
Borrowings
Semi annual interest rate		0.70%
FHLBC advance amount	$ 115,000,000
Total borrowings	$ 115,000,000	$ 70,000,000
FHLB advances at 1.42 %
Borrowings
Semi annual interest rate	1.42%
FHLBC advance amount	$ 110,000,000
FHLB advances at 1.44 %
Borrowings
Semi annual interest rate	1.44%
FHLBC advance amount	$ 5,000,000
Junior subordinated debentures
Borrowings
Total borrowings	$ 57,639,000	57,591,000
Senior Notes.
Borrowings
Semi annual interest rate	5.75%
Debt Instrument, Term	10 years
Interest rate term	5 years
Debt issuance costs	$ 942,000	1,000,000
Total borrowings	$ 44,058,000	$ 43,998,000
Senior Notes. \| Three month LIBOR
Borrowings
Basis points added to reference rate (as a percent)	385.00%

Represents the period for which the interest rate is applicable.

No definition available.

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Represents the maturity term of the debt instrument.

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- Definition

Represents the amount of borrowings from Federal Home Loan Bank as a percentage of total assets.

+ References

No definition available.

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- Definition

Represents the principal balance of loans pledged as collateral in support of federal home loan bank advances outstanding.

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No definition available.

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Represents the fair value of securities pledged as collateral in support of federal home loan bank advances outstanding

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No definition available.

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- Definition

Represents the number of customers having secured balances exceeding specified percentage of stockholders equity.

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No definition available.

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Represents the threshold percentage which the entity uses for disclosure about the customer holding secured balances.

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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No definition available.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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No definition available.

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- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

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No definition available.

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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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- Definition

Amount of advances from the Federal Home Loan Bank (FHLB).

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- Definition

Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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The fair value, as of the date of each statement of financial position presented, of securities which are owned but transferred to serve as collateral for the payment of repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

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The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for repurchase agreements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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