Southern Michigan Bancorp, Inc. (Parent Company Only) Financial Information (Details 2) (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
Operating Activities
Net income	$ 1,167	$ 1,154	$ 1,027	$ 1,002	$ 1,005	$ 887	$ 762	$ 748	$ 4,350	$ 3,402	$ 3,098	$ 1,936	$ 813
Equity in net income, less dividends received, from:
Stock option and restricted stock grant compensation expense									203	168	154
Depreciation									923	869	995
Net change in obligation under ESOP									134	128	140
Other, net									(770)	(707)	(632)
Net cash from operating activities									7,935	8,987	5,529
Financing Activities
Repayments of other borrowings									(6,712)	(2,328)	(5,753)
Cash dividends paid									(760)	(470)	(467)
Stock option exercised									6
Net cash from financing activities									14,253	10,859	28,004
Net change in cash and cash equivalents									8,149	(36,648)	54,019
Beginning cash and cash equivalents				42,185				78,833	42,185	78,833	24,814
Ending cash and cash equivalents	50,334				42,185				50,334	42,185	78,833	24,814
Parent Company [Member]
Operating Activities
Net income									4,350	3,402	3,098
Equity in net income, less dividends received, from:
Subsidiary bank									(2,380)	(2,245)	(2,023)
Non-banking subsidiary									2	1	1
Stock option and restricted stock grant compensation expense									203	168	154
Depreciation									31	32	32
Net change in obligation under ESOP									134	128	140
Other, net									(138)	(84)	(224)
Net cash from operating activities									2,202	1,402	1,178
Financing Activities
Repayments of other borrowings									(1,634)	(1,211)	(644)
Cash dividends paid									(760)	(470)	(467)
Stock option exercised									6
Net cash from financing activities									(2,388)	(1,681)	(1,111)
Net change in cash and cash equivalents									(186)	(279)	67
Beginning cash and cash equivalents				404				683	404	683	616
Ending cash and cash equivalents	$ 218				$ 404				$ 218	$ 404	$ 683