Southern Michigan Bancorp, Inc. (Parent Company Only) Financial Information (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Mar. 31, 2004
Assets
Cash and cash equivalents	$ 50,334	$ 42,185	$ 78,833	$ 24,814
Premises and equipment, net	12,419	12,546
Other assets	7,914	7,830
Total Assets	528,860	509,220	493,880	462,409	474,996
Liabilities and Shareholders' Equity
Other borrowings	1,039	7,751	10,079	10,832	12,492
Subordinated debentures	5,155	5,155				5,155
Common stock subject to repurchase obligation in ESOP	2,065	1,296	1,399	945	728
Shareholders' equity	54,635	51,865	47,843	45,734	44,416
Total Liabilities and Shareholders' Equity	528,860	509,220
Parent Company [Member]
Assets
Cash and cash equivalents	218	404	683	616
Investment in subsidiary bank	60,053	57,947
Investment in non banking subsidiary	186	188
Premises and equipment, net	813	844
Other assets	910	778
Total Assets	62,180	60,161
Liabilities and Shareholders' Equity
Dividends payable	285	165
Other liabilities	40	46
Other borrowings		1,634
Subordinated debentures	5,155	5,155
Common stock subject to repurchase obligation in ESOP	2,065	1,296
Shareholders' equity	54,635	51,865
Total Liabilities and Shareholders' Equity	$ 62,180	$ 60,161