Parent Company Condensed Financial Information - Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Expenses
Junior subordinated debentures interest expense	$ 4,334	$ 4,287	$ 4,919
Interest Expense, Senior Notes	112
Subordinated debt interest expense	949	814	792
Income before income taxes	24,504	24,361	15,897
Income tax benefit	8,820	8,976	5,761
Net income	15,684	15,385	10,136
Dividends waived upon preferred stock redemption			(5,433)
Gain on preferred stock redemption			(1,348)
Net income available to common stockholders	15,684	13,512	11,855
Old Second Bancorp, Inc.
Operating Income
Cash dividends received from subsidiaries		82,777
Other income	130	131	231
Total operating income	130	82,908	231
Operating Expenses
Junior subordinated debentures interest expense	4,334	4,287	4,919
Interest Expense, Senior Notes	112
Subordinated debt interest expense	949	814	792
Other interest expense	8	7	16
Other expenses	1,975	1,978	1,045
Total operating expense	7,378	7,086	6,772
Income before income taxes	(7,248)	75,822	(6,541)
Income tax benefit	(2,909)	(2,771)	(2,305)
Income (loss) before equity in undistributed net income of subsidiaries	(4,339)	78,593	(4,236)
Equity in undistributed net income of subsidiaries	20,023	(63,208)	14,372
Net income	15,684	15,385	10,136
Preferred stock dividends and accretion of discount		1,873	5,062
Dividends waived upon preferred stock redemption			(5,433)
Gain on preferred stock redemption			(1,348)
Net income available to common stockholders	$ 15,684	$ 13,512	$ 11,855