Junior Subordinated Debentures (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended		9 Months Ended	1 Months Ended	9 Months Ended
	Jul. 31, 2003 Old Second Capital Trust I	Jun. 30, 2003 Old Second Capital Trust I	Sep. 30, 2013 Old Second Capital Trust I	Apr. 30, 2007 Old Second Capital Trust II	Sep. 30, 2013 Old Second Capital Trust II
Junior subordinated debentures
Proceeds from sale of cumulative trust preferred securities	$ 4.1	$ 27.5		$ 25.0
Amortization period			30 years		30 years
Maturity Period			30 years		30 years
Cash distribution rate of trust preferred securities (as a percent)			7.80%
Cash distribution fixed rate of trust preferred securities (as a percent)					6.77%
Cash distribution, floating rate base					three-month LIBOR
Basis points added to cash distribution floating rate base (as a percent)					1.50%