Parent Company Condensed Financial Information -Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows from Operating Activities
Net income	$ 15,684	$ 15,385	$ 10,136
Adjustments to reconcile net income (loss) to net cash from operating activities:
Deferred income taxes	8,421	8,756	5,563
Change in taxes payable	260	100	86
Stock-based compensation	657	613	295
Net cash provided by (used in) operating activities	27,255	21,137	(6,307)
Cash Flows from Financing Activities
Dividends paid on preferred stock		(2,417)	(12,390)
Dividends paid common stock	(888)
Purchases of treasury stock	(254)	(117)	(46)
Proceeds from issuance of common stock			64,331
Proceeds From Issuance Of Senior Notes	43,994
Repayment of subordinate debt	(45,000)
Proceeds from (Repayments of) Notes Payable	(500)
Proceeds from Stock Options Exercised	11
Redemption of preferred stock		(47,331)	(24,321)
Net cash (used in) provided by financing activities	102,813	7,200	68,977
Net change in cash and cash equivalents	6,996	(3,859)	(3,463)
Cash and cash equivalents at beginning of period	40,338	44,197	47,660
Cash and cash equivalents at end of period	47,334	40,338	44,197
Old Second Bancorp, Inc.
Cash Flows from Operating Activities
Net income	15,684	15,385	10,136
Adjustments to reconcile net income (loss) to net cash from operating activities:
Equity in undistributed net income of subsidiaries	(20,023)	63,208	(14,372)
Deferred income taxes	(1,039)	(2,806)	(2,256)
Change in taxes payable	330	(199)	22
Change in other assets	171	36	692
Stock-based compensation	657	613	295
Other, net	282	69	(17,103)
Net cash provided by (used in) operating activities	(3,938)	76,306	(22,586)
Cash Flows from Financing Activities
Dividends paid on preferred stock		(2,417)	(12,390)
Dividends paid common stock	(888)
Purchases of treasury stock	(254)	(117)	(46)
Proceeds from issuance of common stock			64,331
Proceeds From Issuance Of Senior Notes	43,994
Repayment of subordinate debt	(45,000)
Proceeds from (Repayments of) Notes Payable	(500)
Proceeds from Stock Options Exercised	11
Redemption of preferred stock		(47,331)	(24,321)
Net cash (used in) provided by financing activities	(2,637)	(49,865)	27,574
Net change in cash and cash equivalents	(6,575)	26,441	4,988
Cash and cash equivalents at beginning of period	33,500	7,059	2,071
Cash and cash equivalents at end of period	$ 26,925	$ 33,500	$ 7,059