Parent Company Condensed Financial Information - Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Other assets	$ 17,674	$ 13,882
Total assets	2,077,863	2,061,787
Liabilities and Stockholders' Equity
Junior subordinated debentures	58,378	58,378
Subordinated debt	45,000	45,000
Other liabilities	9,900	12,655
Stockholders' equity	155,929	194,163	$ 147,692	$ 72,552
Total liabilities and stockholders' equity	2,077,863	2,061,787
Old Second Bancorp, Inc.
Assets
Noninterest bearing deposit with bank subsidiary	33,500	7,059
Investment in subsidiaries	204,509	272,285
Other assets	22,951	19,940
Total assets	260,960	299,284
Liabilities and Stockholders' Equity
Junior subordinated debentures	58,378	58,378
Subordinated debt	45,000	45,000
Other liabilities	1,653	1,743
Stockholders' equity	155,929	194,163
Total liabilities and stockholders' equity	$ 260,960	$ 299,284