Parent Company Condensed Financial Information (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets
Other assets	$ 24,160	$ 34,534
Total assets	2,045,799	1,941,418
Liabilities and Stockholders' Equity
Junior subordinated debentures	58,378	58,378
Subordinated debt	45,000	45,000
Other liabilities	34,275	21,856
Stockholders' equity	72,552	74,002	83,958
Total liabilities and stockholders' equity	2,045,799	1,941,418
Old Second Bancorp, Inc
Assets
Noninterest bearing deposit with bank subsidiary	3,554	4,315
Investment in subsidiaries	192,988	184,058
Other assets	1,712	2,427
Total assets	198,254	190,800
Liabilities and Stockholders' Equity
Junior subordinated debentures	58,378	58,378
Subordinated debt	45,000	45,000
Other liabilities	22,324	13,420
Stockholders' equity	72,552	74,002
Total liabilities and stockholders' equity	$ 198,254	$ 190,800