Basis Of Presentation And Accounting Policies	6 Months Ended
Jun. 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 six months ended June 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. Nonperforming Assets In the second quarter 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, certain current junior lien home equity loans where the Corporation also holds and services the senior position are placed on nonaccrual status when full collection of the senior position is in doubt. 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 9 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 six months ended June 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.

Nonperforming Assets

In the second quarter 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, certain current junior lien home equity loans where the Corporation also holds and services the senior position are placed on nonaccrual status when full collection of the senior position is in doubt.

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 9 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.