Basis of Presentation and Accounting Policies	6 Months Ended
Jun. 30, 2014
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 six months ended June 30, 2014 are not necessarily indicative of the results that may be expected for the year ending December 31, 2014. 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, 2013. Allowance for Credit Losses In the second quarter 2014, the Corporation enhanced the approach used to determine the standard reserve factors used in estimating the allowance for credit losses, which had the effect of capturing certain elements in the standard reserve component that had formerly been included in the qualitative assessment. The impact of the change was largely neutral to the total allowance for loan losses at June 30, 2014. However, because standard reserves are allocated to the segments at the loan level, while qualitative reserves are allocated at the portfolio level, the impact of the methodology change on the allowance of each segment reflected the characteristics of the individual loans within each segment's portfolio, causing segment reserves to increase or decrease accordingly. Recently Adopted Accounting Pronouncement Effective January 1, 2014, the Corporation early adopted Accounting Standards Update (ASU) No. 2014-01, “Investments-Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Qualified Affordable Housing Projects,” an amendment to GAAP which enables companies that invest in affordable housing projects that qualify for the low-income housing tax credit (LIHTC) to elect to use the proportional amortization method if certain conditions are met. Under the proportional amortization method, the initial investment cost of the project is amortized in proportion to the amount of tax credits and other benefits received, with the results of the investment presented on a net basis as a component of the provision for income taxes. Previously, LIHTC investments were accounted for under the cost or equity method, and the amortization was recorded as a reduction to other noninterest income, with the tax credits and other benefits received recorded as a component of the provision for income taxes. The Corporation believes the proportional amortization method better represents the economics of LIHTC investments and provides users with a better understanding of the returns from such investments than the cost or equity method. The cumulative effect of the retrospective application of the change in amortization method was a $3 million decrease to both "accrued income and other assets" and "retained earnings" on the consolidated balance sheets as of January 1, 2013. The unaudited consolidated financial statements have been retrospectively adjusted to reflect the prior period effect of the adoption of the amendment, which resulted in increases of $14 million and $27 million to both "other noninterest income" and "provision for income taxes" for the three- and six-month periods ended June 30, 2013, respectively. The adoption of ASU 2014-01 had no effect on net income or earnings per common share for any period presented. See Note 6 to these unaudited consolidated financial statements for additional information regarding LIHTC and other tax credit investments. Pending Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (FASB) issued ASU No. 2014-09, “Revenue from Contracts with Customers (Topic 606),” (ASU 2014-09), which is intended to improve and converge the financial reporting requirements for revenue contracts with customers. Previous GAAP comprised broad revenue recognition concepts along with numerous industry-specific requirements. The new guidance establishes a five-step model which entities must follow to recognize revenue and removes inconsistencies and weaknesses in existing guidance. ASU 2014-09 is effective for annual and interim periods beginning after December 15, 2016, and must be retrospectively applied. Entities will have the option of presenting prior periods as impacted by the new guidance or presenting the cumulative effect of initial application along with supplementary disclosures. Early adoption is prohibited. The Corporation is currently evaluating the impact of adopting ASU 2014-09. In June 2014, the FASB issued ASU No. 2014-12, “Compensation-Stock Compensation (Topic 718): Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period,” (ASU 2014-12). The new guidance requires that a performance target that affects vesting and that could be achieved after the requisite service period be treated as a performance condition. ASU 2014-12 is effective for annual and interim periods beginning after December 15, 2015, with early adoption permitted. The Corporation's current accounting treatment of performance conditions for employees who are or become retirement eligible prior to the achievement of the performance target are consistent with ASU 2014-12, and as such does not expect the new guidance to have a material effect on the Corporation’s financial condition and results of operations. The Corporation expects to prospectively adopt ASU 2014-12 in the first quarter 2015.

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 six months ended June 30, 2014 are not necessarily indicative of the results that may be expected for the year ending December 31, 2014. 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, 2013.

Allowance for Credit Losses

In the second quarter 2014, the Corporation enhanced the approach used to determine the standard reserve factors used in estimating the allowance for credit losses, which had the effect of capturing certain elements in the standard reserve component that had formerly been included in the qualitative assessment. The impact of the change was largely neutral to the total allowance for loan losses at June 30, 2014. However, because standard reserves are allocated to the segments at the loan level, while qualitative reserves are allocated at the portfolio level, the impact of the methodology change on the allowance of each segment reflected the characteristics of the individual loans within each segment's portfolio, causing segment reserves to increase or decrease accordingly.

Recently Adopted Accounting Pronouncement

Effective January 1, 2014, the Corporation early adopted Accounting Standards Update (ASU) No. 2014-01, “Investments-Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Qualified Affordable Housing Projects,” an amendment to GAAP which enables companies that invest in affordable housing projects that qualify for the low-income housing tax credit (LIHTC) to elect to use the proportional amortization method if certain conditions are met. Under the proportional amortization method, the initial investment cost of the project is amortized in proportion to the amount of tax credits and other benefits received, with the results of the investment presented on a net basis as a component of the provision for income taxes. Previously, LIHTC investments were accounted for under the cost or equity method, and the amortization was recorded as a reduction to other noninterest income, with the tax credits and other benefits received recorded as a component of the provision for income taxes. The Corporation believes the proportional amortization method better represents the economics of LIHTC investments and provides users with a better understanding of the returns from such investments than the cost or equity method.

The cumulative effect of the retrospective application of the change in amortization method was a $3 million decrease to both "accrued income and other assets" and "retained earnings" on the consolidated balance sheets as of January 1, 2013. The unaudited consolidated financial statements have been retrospectively adjusted to reflect the prior period effect of the adoption of the amendment, which resulted in increases of $14 million and $27 million to both "other noninterest income" and "provision for income taxes" for the three- and six-month periods ended June 30, 2013, respectively. The adoption of ASU 2014-01 had no effect on net income or earnings per common share for any period presented.

See Note 6 to these unaudited consolidated financial statements for additional information regarding LIHTC and other tax credit investments.

Pending Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (FASB) issued ASU No. 2014-09, “Revenue from Contracts with Customers (Topic 606),” (ASU 2014-09), which is intended to improve and converge the financial reporting requirements for revenue contracts with customers. Previous GAAP comprised broad revenue recognition concepts along with numerous industry-specific requirements. The new guidance establishes a five-step model which entities must follow to recognize revenue and removes inconsistencies and weaknesses in existing guidance. ASU 2014-09 is effective for annual and interim periods beginning after December 15, 2016, and must be retrospectively applied. Entities will have the option of presenting prior periods as impacted by the new guidance or presenting the cumulative effect of initial application along with supplementary disclosures. Early adoption is prohibited. The Corporation is currently evaluating the impact of adopting ASU 2014-09.

In June 2014, the FASB issued ASU No. 2014-12, “Compensation-Stock Compensation (Topic 718): Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period,” (ASU 2014-12). The new guidance requires that a performance target that affects vesting and that could be achieved after the requisite service period be treated as a performance condition. ASU 2014-12 is effective for annual and interim periods beginning after December 15, 2015, with early adoption permitted. The Corporation's current accounting treatment of performance conditions for employees who are or become retirement eligible prior to the achievement of the performance target are consistent with ASU 2014-12, and as such does not expect the new guidance to have a material effect on the Corporation’s financial condition and results of operations. The Corporation expects to prospectively adopt ASU 2014-12 in the first quarter 2015.