Summary of Significant Accounting Policies	3 Months Ended
Mar. 31, 2012
Summary of Significant Accounting Policies and New Accounting Pronouncements [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	NOTE A – SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Principles of Consolidation: The consolidated financial statements include the accounts of Southern Michigan Bancorp, Inc. (the Company) and its wholly-owned subsidiary, Southern Michigan Bank & Trust (SMB&T), after elimination of significant inter-company balances and transactions. SMB&T owns FNB Financial Services, which conducts a brokerage business and is consolidated into SMB&T’s financial statements. During 2004, the Company formed a special purpose trust, Southern Michigan Bancorp Capital Trust I, for the sole purpose of issuing trust preferred securities. Under accounting principles generally accepted in the United States of America, the trust is not consolidated into the financial statements of the Company. The accompanying unaudited consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America for interim financial information and with the instructions for Form 10-Q and Rule 8-03 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by accounting principles generally accepted in the United States of America for complete financial statements. In the opinion of management, all adjustments considered necessary in order to make the financial statements not misleading have been included. Operating results for the three months ended March 31, 2012 are not necessarily indicative of the results that may be expected for the year ending December 31, 2012. The accompanying consolidated financial statements should be read in conjunction with the consolidated financial statements and related footnotes of the Company for December 31, 2011 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2011, filed with the Securities and Exchange Commission on March 30, 2012. Loans: Loans are reported at the principal balance outstanding, net of unearned interest, deferred loan fees and costs, and an allowance for loan losses. Interest income is reported on the interest method and includes amortization of net deferred loan fees and costs over the loan term. Interest income is not reported when full loan repayment is in doubt, typically when payments are past due over 90 days, unless the loan is both well secured and in the process of collection. Past due status is based on the contractual terms of the loan. All interest accrued but not received for these loans is reversed against interest income. Payments received on such loans are reported as principal reductions until qualifying for return to accrual. Loans are returned to accrual status when all the principal and interest contractually due are brought current and future payments are reasonably assured. Allowance for Loan Losses: The allowance for loan losses is a valuation allowance for probable incurred credit losses, increased by the provision for loan losses and decreased by charge-offs less recoveries. Estimating the risk of loss and the amount of loss on any loan is necessarily subjective. Accordingly, management estimates the allowance balance based on past loan loss experience, nature and volume of the portfolio, information about specific borrower situations and estimated collateral values, economic conditions, information in regulatory examination reports, and other factors. Allocations of the allowance may be made for specific loans, but the entire allowance is available for any loan that, in management’s judgment, should be charged-off. Loan losses are charged against the allowance when management believes the uncollectibility of a loan balance is confirmed. The allowance consists of specific and general components. The specific component relates to loans that are individually classified as impaired. Consumer loans are typically charged-off no later than 120 days past due. Real estate mortgage loans in the process of collection are charged-off on or before they become 365 days past due. Commercial loans are charged-off promptly upon the determination that all or a portion of any loan balance is uncollectible. In all cases, loans are placed on nonaccrual status or charged-off at an earlier date if collection of principal or interest is considered doubtful. Impaired Loans: Loan impairment is reported when full payment under the loan terms is not expected. Impairment is evaluated in total for smaller-balance loans of similar nature such as residential mortgage and consumer loans and on an individual loan basis for other loans. If a loan is impaired, a portion of the allowance is allocated so that the loan is reported, net, at the present value of estimated future cash flows using the loan’s effective interest rate or at the fair value of collateral if repayment is expected solely from the collateral. Loans are evaluated for impairment when payments are delayed, typically 90 days or more, or when it is probable that all principal and interest amounts will not be collected according to the contractual terms of the loan. Loans for which the terms have been modified and for which the borrower is experiencing financial difficulties, are considered troubled debt restructurings and are classified as impaired. All nonaccrual loans have also been determined by the Company to meet the definition of an impaired loan. Reclassifications Certain 2011 items have been reclassified to conform to the 2012 presentation.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2012

Summary of Significant Accounting Policies and New Accounting Pronouncements [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

NOTE A – SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Principles of Consolidation: The consolidated financial statements include the accounts of Southern Michigan Bancorp, Inc. (the Company) and its wholly-owned subsidiary, Southern Michigan Bank & Trust (SMB&T), after elimination of significant inter-company balances and transactions. SMB&T owns FNB Financial Services, which conducts a brokerage business and is consolidated into SMB&T’s financial statements. During 2004, the Company formed a special purpose trust, Southern Michigan Bancorp Capital Trust I, for the sole purpose of issuing trust preferred securities. Under accounting principles generally accepted in the United States of America, the trust is not consolidated into the financial statements of the Company.

The accompanying unaudited consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America for interim financial information and with the instructions for Form 10-Q and Rule 8-03 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by accounting principles generally accepted in the United States of America for complete financial statements. In the opinion of management, all adjustments considered necessary in order to make the financial statements not misleading have been included. Operating results for the three months ended March 31, 2012 are not necessarily indicative of the results that may be expected for the year ending December 31, 2012.

The accompanying consolidated financial statements should be read in conjunction with the consolidated financial statements and related footnotes of the Company for December 31, 2011 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2011, filed with the Securities and Exchange Commission on March 30, 2012.

Loans: Loans are reported at the principal balance outstanding, net of unearned interest, deferred loan fees and costs, and an allowance for loan losses. Interest income is reported on the interest method and includes amortization of net deferred loan fees and costs over the loan term. Interest income is not reported when full loan repayment is in doubt, typically when payments are past due over 90 days, unless the loan is both well secured and in the process of collection. Past due status is based on the contractual terms of the loan. All interest accrued but not received for these loans is reversed against interest income. Payments received on such loans are reported as principal reductions until qualifying for return to accrual. Loans are returned to accrual status when all the principal and interest contractually due are brought current and future payments are reasonably assured.

Allowance for Loan Losses: The allowance for loan losses is a valuation allowance for probable incurred credit losses, increased by the provision for loan losses and decreased by charge-offs less recoveries. Estimating the risk of loss and the amount of loss on any loan is necessarily subjective. Accordingly, management estimates the allowance balance based on past loan loss experience, nature and volume of the portfolio, information about specific borrower situations and estimated collateral values, economic conditions, information in regulatory examination reports, and other factors. Allocations of the allowance may be made for specific loans, but the entire allowance is available for any loan that, in management’s judgment, should be charged-off. Loan losses are charged against the allowance when management believes the uncollectibility of a loan balance is confirmed. The allowance consists of specific and general components. The specific component relates to loans that are individually classified as impaired.

Consumer loans are typically charged-off no later than 120 days past due. Real estate mortgage loans in the process of collection are charged-off on or before they become 365 days past due. Commercial loans are charged-off promptly upon the determination that all or a portion of any loan balance is uncollectible. In all cases, loans are placed on nonaccrual status or charged-off at an earlier date if collection of principal or interest is considered doubtful.

Impaired Loans: Loan impairment is reported when full payment under the loan terms is not expected. Impairment is evaluated in total for smaller-balance loans of similar nature such as residential mortgage and consumer loans and on an individual loan basis for other loans. If a loan is impaired, a portion of the allowance is allocated so that the loan is reported, net, at the present value of estimated future cash flows using the loan’s effective interest rate or at the fair value of collateral if repayment is expected solely from the collateral. Loans are evaluated for impairment when payments are delayed, typically 90 days or more, or when it is probable that all principal and interest amounts will not be collected according to the contractual terms of the loan. Loans for which the terms have been modified and for which the borrower is experiencing financial difficulties, are considered troubled debt restructurings and are classified as impaired. All nonaccrual loans have also been determined by the Company to meet the definition of an impaired loan.

Reclassifications

Certain 2011 items have been reclassified to conform to the 2012 presentation.