Borrowings - Additional information (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Borrowings
Borrowings at FHLBC as percentage of total assets	35.00%
Bank owned FHLBC stock	$ 3,100,000	$ 3,700,000
Fair value of securities collateralized	63,300,000
Principal balance of loans collateralized	168,200,000
Maximum borrowing capacity	158,000,000
Amount available for additional borrowings	51,700,000
Senior notes	43,998,000
Total borrowings	197,304,000	152,113,000
Securities sold under repurchase agreements
Borrowings
Carrying amount of securities secured	25,700,000	34,100,000
Fair value of the pledged collateral	$ 43,000,000	45,400,000
Number of customers having secured balances exceeding specified percentage of stockholders equity | item	0
Total borrowings	$ 25,715,000	34,070,000
Additional information related to repurchase agreements
Average daily balance during the year	$ 34,016,000	$ 28,194,000	$ 26,093,000
Average interest rate during the year (as a percent)	0.01%	0.01%	0.01%
Maximum month-end balance during the year	$ 46,606,000	$ 34,785,000	$ 38,133,000
Weighted average interest rate at year-end (as a percent)	0.01%	0.01%	0.01%
Securities sold under repurchase agreements | Minimum
Borrowings
Maturity	1 day
Securities sold under repurchase agreements | Maximum
Borrowings
Maturity	90 days
FHLBC advances
Borrowings
Interest rate	0.70%
FHLBC advance amount	$ 70,000,000
Total borrowings	$ 70,000,000	$ 15,000,000
Subordinated debt
Borrowings
Basis points added to reference rate (as a percent)	150.00%
Total borrowings	$ 45,000,000	45,000,000
Notes payable and other borrowings
Borrowings
Total borrowings		500,000
Senior debt facility | LIBOR
Borrowings
Basis points added to reference rate (as a percent)	90.00%
Term debt
Borrowings
Total borrowings	$ 500,000
Junior subordinated debentures
Borrowings
Total borrowings	$ 57,591,000	$ 57,543,000
Senior Notes.
Borrowings
Interest rate	5.75%
Debt Instrument, Term	10 years
Interest rate term	5 years
Senior note debt issuance costs	$ 1,000,000
Total borrowings	$ 43,998,000
Senior Notes. | LIBOR
Borrowings
Basis points added to reference rate (as a percent)	385.00%