Basis Of Presentation And Accounting Policies	9 Months Ended
Sep. 30, 2012
Basis Of Presentation And Accounting Policies [Abstract]
Basis of Presentation and Accounting Policies	BASIS OF PRESENTATION AND ACCOUNTING POLICIES The accompanying unaudited consolidated financial statements were prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, the statements do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation were included. The results of operations for the nine months ended September 30, 2012 are not necessarily indicative of the results that may be expected for the year ending December 31, 2012. Certain items in prior periods were reclassified to conform to the current presentation. For further information, refer to the consolidated financial statements and footnotes thereto included in the Annual Report of Comerica Incorporated and Subsidiaries (the Corporation) on Form 10-K for the year ended December 31, 2011. Beginning in the second quarter 2012, the Corporation presents the provision for loan losses and the provision for credit losses on lending-related commitments together as the "provision for credit losses" on the consolidated statements of comprehensive income. Prior period amounts were reclassified to conform to the current presentation. Allowance for Loan Losses In the first quarter 2012, the Corporation implemented enhancements to the methodology used for determining standard reserve factors for business loans not individually evaluated, which resulted in a $25 million increase to the allowance for loan losses in the first quarter 2012. The enhancements included (a) estimating probability of default and loss given default from a national perspective, in addition to a market-by-market basis, and (b) expanding the time horizon of historical, migration-based probability of default and loss given default experience used to develop the standard reserve factors for each internal risk rating. Nonperforming Assets In the second quarter of 2012, the Corporation modified its residential mortgage and home equity nonaccrual policy. Under the new policy, residential mortgage and home equity loans are generally placed on nonaccrual status once they become 90 days past due (previously no later than 180 days past due). In addition, effective second quarter 2012, the Corporation placed on nonaccrual status certain current junior lien home equity loans in which the Corporation holds and services the senior position, when full collection of the senior position is in doubt. In the third quarter 2012, the Corporation applied the same policy to junior lien home equity loans held by the Corporation for which the senior liens are serviced by others. In the third quarter 2012, in connection with recently issued regulatory guidance, the Corporation further modified its nonaccrual and charge-off policy regarding Chapter 7 bankruptcy residential mortgage and consumer loans for which the court has discharged the borrower's obligation and the borrower has not reaffirmed the debt. Effective September 30, 2012, such loans are placed on nonaccrual status and written down to estimated collateral value, without regard to the actual payment status of the loan, and are classified as troubled debt restructurings (TDRs). These modifications resulted in $4 million of write-downs and $8 million of nonaccrual TDRs. Of the $8 million of nonaccrual TDRs, $6 million were previously considered nonaccrual loans, therefore the net increase to nonperforming assets was $2 million. Recently Adopted Accounting Standards In the first quarter 2012, the Corporation adopted amendments to GAAP which revise the presentation of comprehensive income in the financial statements. As a result, the Corporation presents on an interim basis the components of net income and a total for comprehensive income in one continuous consolidated statement of comprehensive income and will present on an annual basis the components of net income and other comprehensive income in two separate, but consecutive statements. Information on the components of other comprehensive income is provided on an interim basis in Note 8 to these unaudited financial statements. In the first quarter 2012, the Corporation adopted an amendment to GAAP which generally aligns the principles of fair value measurements with International Financial Reporting Standards (IFRS) and requires expanded disclosures. The adoption of the amendment had no impact on the Corporation's financial condition or results of operations. The required disclosures are provided in Note 2 to these unaudited financial statements.

BASIS OF PRESENTATION AND ACCOUNTING POLICIES

The accompanying unaudited consolidated financial statements were prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, the statements do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation were included. The results of operations for the nine months ended September 30, 2012 are not necessarily indicative of the results that may be expected for the year ending December 31, 2012. Certain items in prior periods were reclassified to conform to the current presentation. For further information, refer to the consolidated financial statements and footnotes thereto included in the Annual Report of Comerica Incorporated and Subsidiaries (the Corporation) on Form 10-K for the year ended December 31, 2011.

Beginning in the second quarter 2012, the Corporation presents the provision for loan losses and the provision for credit losses on lending-related commitments together as the "provision for credit losses" on the consolidated statements of comprehensive income. Prior period amounts were reclassified to conform to the current presentation.

Allowance for Loan Losses

In the first quarter 2012, the Corporation implemented enhancements to the methodology used for determining standard reserve factors for business loans not individually evaluated, which resulted in a $25 million increase to the allowance for loan losses in the first quarter 2012. The enhancements included (a) estimating probability of default and loss given default from a national perspective, in addition to a market-by-market basis, and (b) expanding the time horizon of historical, migration-based probability of default and loss given default experience used to develop the standard reserve factors for each internal risk rating.

Nonperforming Assets

In the second quarter of 2012, the Corporation modified its residential mortgage and home equity nonaccrual policy. Under the new policy, residential mortgage and home equity loans are generally placed on nonaccrual status once they become 90 days past due (previously no later than 180 days past due).

In addition, effective second quarter 2012, the Corporation placed on nonaccrual status certain current junior lien home equity loans in which the Corporation holds and services the senior position, when full collection of the senior position is in doubt. In the third quarter 2012, the Corporation applied the same policy to junior lien home equity loans held by the Corporation for which the senior liens are serviced by others.

In the third quarter 2012, in connection with recently issued regulatory guidance, the Corporation further modified its nonaccrual and charge-off policy regarding Chapter 7 bankruptcy residential mortgage and consumer loans for which the court has discharged the borrower's obligation and the borrower has not reaffirmed the debt. Effective September 30, 2012, such loans are placed on nonaccrual status and written down to estimated collateral value, without regard to the actual payment status of the loan, and are classified as troubled debt restructurings (TDRs). These modifications resulted in $4 million of write-downs and $8 million of nonaccrual TDRs. Of the $8 million of nonaccrual TDRs, $6 million were previously considered nonaccrual loans, therefore the net increase to nonperforming assets was $2 million.

Recently Adopted Accounting Standards

In the first quarter 2012, the Corporation adopted amendments to GAAP which revise the presentation of comprehensive income in the financial statements. As a result, the Corporation presents on an interim basis the components of net income and a total for comprehensive income in one continuous consolidated statement of comprehensive income and will present on an annual basis the components of net income and other comprehensive income in two separate, but consecutive statements. Information on the components of other comprehensive income is provided on an interim basis in Note 8 to these unaudited financial statements.

In the first quarter 2012, the Corporation adopted an amendment to GAAP which generally aligns the principles of fair value measurements with International Financial Reporting Standards (IFRS) and requires expanded disclosures. The adoption of the amendment had no impact on the Corporation's financial condition or results of operations. The required disclosures are provided in Note 2 to these unaudited financial statements.