Junior Subordinated Debentures - Issuance (Details) - USD ($) $ in Thousands	1 Months Ended				6 Months Ended
	Jun. 30, 2017	Apr. 30, 2007	Jul. 31, 2003	Jun. 30, 2003	Jun. 30, 2017	Dec. 31, 2016
Junior subordinated debentures
Junior subordinated debentures	$ 57,615				$ 57,615	$ 57,591
Old Second Capital Trust I
Junior subordinated debentures
Proceeds from sale of cumulative trust preferred securities			$ 4,100	$ 27,500
Maturity Period					30 years
Cash distribution rate of trust preferred securities (as a percent)					7.80%
Old Second Capital Trust II
Junior subordinated debentures
Proceeds from sale of cumulative trust preferred securities		$ 25,000
Maturity Period					30 years
Old Second Capital Trust II | Debt Instrument, Redemption, Period One [Member]
Junior subordinated debentures
Cash distribution rate of trust preferred securities (as a percent)	4.34%
Cash distribution fixed rate of trust preferred securities (as a percent)					6.77%
Old Second Capital Trust II | Debt Instrument, Redemption, Period Two [Member]
Junior subordinated debentures
Basis points added to cash distribution floating rate base (as a percent)					1.50%
Old Second Capital Trust II | Debt Instrument, Redemption, Period Two [Member] | Three month LIBOR
Junior subordinated debentures
Cash distribution, floating rate base					three-month LIBOR
Junior subordinated debentures | Old Second Capital Trust I
Junior subordinated debentures
Junior subordinated debentures	$ 32,600				$ 32,600
Junior subordinated debentures | Old Second Capital Trust II
Junior subordinated debentures
Junior subordinated debentures	$ 25,800				$ 25,800