Southern Michigan Bancorp, Inc. (Parent Company Only) Financial Information (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Schedule of Comprehensive Income (Loss)
Interest expense									$ (2,817)	$ (3,751)	$ (4,470)
Equity in net income, less dividends received, from:
Net Income	1,167	1,154	1,027	1,002	1,005	887	762	748	4,350	3,402	3,098	1,936	813
Parent Company [Member]
Schedule of Comprehensive Income (Loss)
Dividends from subsidiary bank									2,254	1,437	1,358
Interest income									7	16	25
Interest expense									(228)	(266)	(300)
Rental income from subsidiary bank									137	137	137
Other expenses									(344)	(310)	(290)
Net Income (Loss) Attributable to Noncontrolling Interest, Total									1,826	1,014	930
Federal income tax credit									(146)	(144)	(146)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total									1,972	1,158	1,076
Equity in net income, less dividends received, from:
Subsidiary bank									2,380	2,245	2,023
Non-banking subsidiary									(2)	(1)	(1)
Net Income									$ 4,350	$ 3,402	$ 3,098