Basis of Presentation and Accounting Policies	9 Months Ended
Sep. 30, 2016
Accounting Policies [Abstract]
Basis of Presentation and Accounting Policies	BASIS OF PRESENTATION AND ACCOUNTING POLICIES Organization The accompanying unaudited consolidated financial statements were prepared in accordance with United States (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, 2016 are not necessarily indicative of the results that may be expected for the year ending December 31, 2016. 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, 2015. Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2016-02, “Leases (Topic 842),” (ASU 2016-02), to increase the transparency and comparability of lease recognition and disclosure. The update requires lessees to recognize lease contracts with a term greater than one year on the balance sheet, while recognizing expenses on the income statement in a manner similar to current guidance. For lessors, the update makes targeted changes to the classification criteria and the lessor accounting model to align the guidance with the new lessee model and revenue guidance. ASU 2016-02 is effective for the Corporation on January 1, 2019 and must be applied using the modified retrospective approach. Early adoption is permitted. The Corporation is currently evaluating the impact of adopting ASU 2016-02. In March 2016, the FASB issued ASU No. 2016-09, “Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payments Accounting,” (ASU 2016-09), which intends to simplify accounting for share based payment transactions, including the income tax consequences and classification of awards. Among other items, the update requires excess tax benefits and deficiencies to be recognized as a component of income taxes within the income statement rather than other comprehensive income as required in current guidance. ASU 2016-09 is effective for the Corporation on January 1, 2017. The recognition of excess tax benefits and deficiencies in the income statement must be adopted prospectively. The method of transition required will differ for other items being amended. Early adoption is permitted. The impact to the Corporation upon adoption is dependent on the market value per share of the Corporation's common stock at option expiration dates and restricted stock vesting dates. In June 2016, the FASB issued ASU No. 2016-13, "Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments," (ASU 2016-13), which addresses concerns regarding the perceived delay in recognition of credit losses under the existing incurred loss model. The amendment introduces a new, single model for recognizing credit losses on all financial instruments presented on cost basis. Under the new model, entities must estimate current expected credit losses by considering all available relevant information, including historical and current information, as well as reasonable and supportable forecasts of future events. The update also requires additional qualitative and quantitative information to allow users to better understand the credit risk within the portfolio and the methodologies for determining allowance. ASU 2016-13 is effective for the Corporation on January 1, 2020 and must be applied using the modified retrospective approach with limited exceptions. Early adoption is permitted. The Corporation is currently evaluating the impact of adopting ASU 2016-13. In August 2016, the FASB issued ASU No. 2016-15, “Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments,” (ASU 2016-15), which reduces diversity in the presentation of several categories of transactions in the cash flow statement. Among other things, the update clarifies the appropriate classification for proceeds from settlement of bank owned life insurance (BOLI) policies. Based on preliminary assessments, the Corporation expects to change the classification of proceeds from settlement of BOLI policies from operating activities to investing activities. Proceeds from settlement of BOLI policies totaled $10 million and $6 million for the nine-month periods ended September 30, 2016 and 2015, respectively, and $12 million for the year ended December 31, 2015. Other changes in classification resulting from this update are not expected to be significant. ASU 2016-15 is effective for the Corporation on January 1, 2018 and must be applied using the retrospective approach. Early adoption is permitted.

BASIS OF PRESENTATION AND ACCOUNTING POLICIES

Organization

The accompanying unaudited consolidated financial statements were prepared in accordance with United States (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, 2016 are not necessarily indicative of the results that may be expected for the year ending December 31, 2016. 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, 2015.

Recently Issued Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2016-02, “Leases (Topic 842),” (ASU 2016-02), to increase the transparency and comparability of lease recognition and disclosure. The update requires lessees to recognize lease contracts with a term greater than one year on the balance sheet, while recognizing expenses on the income statement in a manner similar to current guidance. For lessors, the update makes targeted changes to the classification criteria and the lessor accounting model to align the guidance with the new lessee model and revenue guidance. ASU 2016-02 is effective for the Corporation on January 1, 2019 and must be applied using the modified retrospective approach. Early adoption is permitted. The Corporation is currently evaluating the impact of adopting ASU 2016-02.

In March 2016, the FASB issued ASU No. 2016-09, “Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payments Accounting,” (ASU 2016-09), which intends to simplify accounting for share based payment transactions, including the income tax consequences and classification of awards. Among other items, the update requires excess tax benefits and deficiencies to be recognized as a component of income taxes within the income statement rather than other comprehensive income as required in current guidance. ASU 2016-09 is effective for the Corporation on January 1, 2017. The recognition of excess tax benefits and deficiencies in the income statement must be adopted prospectively. The method of transition required will differ for other items being amended. Early adoption is permitted. The impact to the Corporation upon adoption is dependent on the market value per share of the Corporation's common stock at option expiration dates and restricted stock vesting dates.

In June 2016, the FASB issued ASU No. 2016-13, "Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments," (ASU 2016-13), which addresses concerns regarding the perceived delay in recognition of credit losses under the existing incurred loss model. The amendment introduces a new, single model for recognizing credit losses on all financial instruments presented on cost basis. Under the new model, entities must estimate current expected credit losses by considering all available relevant information, including historical and current information, as well as reasonable and supportable forecasts of future events. The update also requires additional qualitative and quantitative information to allow users to better understand the credit risk within the portfolio and the methodologies for determining allowance. ASU 2016-13 is effective for the Corporation on January 1, 2020 and must be applied using the modified retrospective approach with limited exceptions. Early adoption is permitted. The Corporation is currently evaluating the impact of adopting ASU 2016-13.

In August 2016, the FASB issued ASU No. 2016-15, “Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments,” (ASU 2016-15), which reduces diversity in the presentation of several categories of transactions in the cash flow statement. Among other things, the update clarifies the appropriate classification for proceeds from settlement of bank owned life insurance (BOLI) policies. Based on preliminary assessments, the Corporation expects to change the classification of proceeds from settlement of BOLI policies from operating activities to investing activities. Proceeds from settlement of BOLI policies totaled $10 million and $6 million for the nine-month periods ended September 30, 2016 and 2015, respectively, and $12 million for the year ended December 31, 2015. Other changes in classification resulting from this update are not expected to be significant. ASU 2016-15 is effective for the Corporation on January 1, 2018 and must be applied using the retrospective approach. Early adoption is permitted.