Basis of Presentation and Accounting Policies	9 Months Ended
Sep. 30, 2020
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, 2020 are not necessarily indicative of the results that may be expected for the year ending December 31, 2020. 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, 2019. Allowance for Credit Losses The allowance for credit losses includes both the allowance for loan losses and the allowance for credit losses on lending-related commitments. The Corporation disaggregates the loan portfolio into segments for purposes of determining the allowance for credit losses. These segments are based on the level at which the Corporation develops, documents and applies a systematic methodology to determine the allowance for credit losses. The Corporation's portfolio segments are business loans and retail loans. Business loans include the commercial, real estate construction, commercial mortgage, lease financing and international loan portfolios. Retail loans consist of residential mortgage and consumer loans, including home equity loans. Effective January 1, 2020, the Corporation adopted the provisions of Accounting Standards Update (ASU) No. 2016-13, "Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments," commonly referred to as the current expected credit loss (CECL) model, for all financial assets, except those accounted for at fair value through net income, using the modified retrospective approach. Under Topic 326, current expected credit losses are estimated over the contractual life of the loan portfolio, considering all available relevant information, including historical and current conditions as well as reasonable and supportable forecasts of future events. The previous accounting principle estimated probable, estimable losses inherent in the portfolio. At adoption, the Corporation’s estimate of current expected credit losses in accordance with Topic 326 resulted in a $17 million day-one decrease in the overall allowance for credit losses, from $668 million at December 31, 2019 under the incurred loss model. The Corporation recognized a corresponding $13 million increase to retained earnings and a $4 million reduction to deferred tax assets. A similar adjustment at December 31, 2019 would have caused a 2-basis-point increase in the Common Equity Tier 1 (CET1) capital ratio. Business loans, comprising approximately 91 percent of the Corporation’s total loan portfolio at transition, consist of loans and lending arrangements with generally short contractual maturities. As a result, the allowance for credit losses for business loans decreased $42 million. The allowance for credit losses increased $25 million for retail loans, given their longer contractual maturities. Topic 326 also requires expected credit losses on available-for-sale (AFS) debt securities to be recorded as an allowance for credit losses. For certain types of debt securities, such as U.S. Treasuries and other securities with government guarantees, entities may expect zero credit losses. The zero-loss expectation applies to all the Corporation’s securities and no allowance for credit losses was recorded on its AFS securities portfolio at transition. Allowance for Loan Losses The allowance for loan losses is estimated on a quarterly basis and represents management’s estimates of current expected credit losses in the Corporation’s loan portfolio. Pools of loans with similar risk characteristics are collectively evaluated while loans that no longer share risk characteristics with loan pools are evaluated individually. Collective loss estimates are determined by applying reserve factors, designed to estimate current expected credit losses, to amortized cost balances over the remaining contractual life of the collectively evaluated portfolio. Loans with similar risk characteristics are aggregated into homogeneous pools. Business loans are assigned to pools based primarily on business line and the Corporation’s internal risk rating system. For retail loans, pools are based on loan type, past due status and credit scores. Reserve factors are based on estimated probability of default for each pool, set to a default horizon based on contractual life, and loss given default. Historical estimates are calibrated to economic forecasts over the reasonable and supportable forecast period based on the projected performance of specific economic variables that statistically correlate with each of the probability of default and loss given default pools. The calculation of current expected credit losses is inherently subjective, as it requires management to exercise judgment in determining appropriate factors used to determine the allowance. Some of the most significant factors in the quantitative allowance estimate are assigning internal risk ratings to loans, selecting the economic forecasts used to calibrate the reserve factors and determining the reasonable and supportable forecast period. •Internal Risk Ratings: Loss factors are dependent on loan risk ratings for business loans. Risk ratings are assigned at origination, based on inherent credit risk, and may be updated based on new information that becomes available, periodic reviews of credit quality, a change in borrower performance or modifications to lending agreements. •Economic Forecasts: Management selects economic variables it believes to be most relevant based on the composition of the loan portfolio and customer base, including forecasted levels of employment, gross domestic product, corporate bond and treasury spreads, industrial production levels, consumer and commercial real estate price indices as well as housing statistics. Different economic forecast scenarios ranging from more benign to more severe are evaluated each reporting period to forecast losses over the contractual life of the loan portfolio. •Forecast Period: Economic forecasts are applied over the period management believes it can estimate reasonable and supportable forecasts. Forecast periods may be adjusted in response to changes in the economic environment. To estimate losses for contractual periods that extend beyond the forecast horizon, the Corporation reverts to an average historical loss experience. The Corporation typically forecasts economic variables over a two-year horizon, followed by an immediate reversion to an average historical loss experience that generally incorporates a full economic cycle. Management reviews this methodology on at least an annual basis. The allowance for loan losses also includes qualitative adjustments to bring the allowance to the level management believes is appropriate based on factors that have not otherwise been fully accounted for, including adjustments for foresight risk, input imprecision and model imprecision. Foresight risk reflects the inherent imprecision in forecasting economic variables, including determining the depth and duration of economic cycles and their impact to relevant economic variables. The Corporation may make qualitative adjustments based on its evaluation of different forecast scenarios and known recent events impacting relevant economic variables. Input imprecision factors address the risk that certain model inputs may not reflect all available information including (i) risk factors that have not been fully addressed in internal risk ratings, (ii) changes in lending policies and procedures, (iii) changes in the level and quality of experience held by lending management, (iv) imprecision in the risk rating system and (v) limitations in data available for certain loan portfolios. Model imprecision considers known model limitations and model updates not yet fully reflected in the quantitative estimate. The determination of the appropriate qualitative adjustment is based on management's analysis of current and expected economic conditions and their impact to the portfolio, as well as internal credit risk movements and a qualitative assessment of the lending environment, including underwriting standards. Management recognizes the sensitivity of various assumptions made in the quantitative modeling of expected losses and may adjust reserves depending upon the level of uncertainty that currently exists in one or more assumptions. Credit losses for loans that no longer share risk characteristics with the loan pools are estimated on an individual basis. Individual credit loss estimates are typically performed for nonaccrual loans and modified loans classified as troubled debt restructurings (TDRs) and are based on one of several methods, including the estimated fair value of the underlying collateral, observable market value of similar debt or the present value of expected cash flows. The Corporation considers certain loans to be collateral-dependent if the borrower is experiencing financial difficulty and management expects repayment for the loan to be substantially through the operation or sale of the collateral. For collateral-dependent loans, loss estimates are based on the fair value of collateral, less estimated cost to sell (if applicable). Collateral values supporting individually evaluated loans are assessed quarterly and appraisals are typically obtained at least annually. The total allowance for loan losses is sufficient to absorb expected credit losses over the contractual life of the portfolio. Unanticipated events impacting the economy, including political instability or global events affecting the U.S. economy, could cause changes to expectations for current conditions and economic forecasts that result in an unanticipated increase in the allowance. Significant increases in current portfolio exposures or changes in credit characteristics could also increase the amount of the allowance. Such events, or others of similar nature, may result in the need for additional provision for credit losses in order to maintain an allowance that complies with credit risk and accounting policies. Loans deemed uncollectible are charged off and deducted from the allowance. Recoveries on loans previously charged off are added to the allowance. Credit losses are not estimated for accrued interest receivable as interest that is deemed uncollectible is written off through interest income. Allowance for Credit Losses on Lending-Related Commitments The allowance for credit losses on lending-related commitments estimates current expected credit losses on collective pools of letters of credit and unused commitments to extend credit based on reserve factors, determined in a manner similar to business loans, multiplied by a probability of draw estimate, based on historical experience and credit risk, applied to commitment amounts. The allowance for credit losses on lending-related commitments is included in accrued expenses and other liabilities on the Consolidated Balance Sheets, with the corresponding charge included in the provision for credit losses on the Consolidated Statements of Comprehensive Income. Goodwill Effective January 1, 2020, the Corporation prospectively adopted the provisions of ASU No. 2017-04, “Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment,” (ASU 2017-04), eliminating the second step of goodwill impairment testing, under which the implied fair value of goodwill was determined as if the reporting unit were being acquired in a business combination. Under ASU 2017-04, the Corporation will compare the fair value of a reporting unit with its carrying amount and recognize an impairment charge for any amount by which the carrying amount exceeds the reporting unit’s fair value, to the extent that the loss recognized does not exceed the amount of goodwill allocated to that reporting unit. The adoption of ASU 2017-04 did not impact the Corporation’s current financial condition or results of operations. Goodwill, included in accrued income and other assets on the Consolidated Balance Sheets, is initially recorded as the excess of the purchase price over the fair value of net assets acquired in a business combination and is subsequently evaluated at least annually for impairment. Goodwill impairment testing is performed at the reporting unit level, equivalent to a business segment or one level below. The Corporation has three reporting units: the Commercial Bank, the Retail Bank and Wealth Management. The Corporation performs its annual evaluation of goodwill impairment in the third quarter of each year and may elect to perform a quantitative impairment analysis or first conduct a qualitative analysis to determine if a quantitative analysis is necessary. Additionally, the Corporation evaluates goodwill impairment on an interim basis if events or changes in circumstances between annual tests indicate additional testing may be warranted to determine if goodwill might be impaired. Factors considered in the assessment of the likelihood of impairment include macroeconomic conditions, industry and market considerations, stock performance of the Corporation and its peers, financial performance of the reporting units, and previous results of goodwill impairment tests, amongst other factors. Based on the results of the qualitative analysis, the Corporation determines whether a quantitative test is deemed necessary. The quantitative test compares the estimated fair value of identified reporting units with their carrying amount, including goodwill. If the estimated fair value of the reporting unit is less than the carrying value, an impairment charge would be recorded for the excess, not to exceed the amount of goodwill allocated to the reporting unit. Software Effective January 1, 2020, the Corporation adopted the provisions of ASU No. 2018-15, “Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract," using a prospective approach for implementation costs incurred subsequent to the adoption of the standard. Amortization expense from capitalized implementation costs of hosting arrangements that are service contracts and fees associated with the hosting elements of the arrangements, are included in software expense in the Consolidated Statements of Comprehensive Income, along with other software-related expenses. For the three- and nine-month periods ended September 30, 2020, software expense included $9 million and $24 million of hosting fees that would have previously been included in outside processing fee expense. Capitalized software, stated at cost less accumulated amortization, includes purchased software, capitalizable application development costs associated with internally developed software and cloud computing arrangements, including capitalizable implementation costs associated with hosting arrangements that are service contracts. Amortization, computed on the straight-line method, is charged to software expense in the Consolidated Statements of Comprehensive Income over the estimated useful life of the software, generally 5 years, or the term of the hosting arrangement for implementation costs related to service contracts. Cloud computing arrangements include software as a service (SaaS), platform as a service (PaaS), infrastructure as a service (IaaS) and other similar hosting arrangements. The Corporation primarily utilizes SaaS and IaaS arrangements. Capitalized implementation costs of hosting arrangements that are service contracts were $3 million for both the three- and nine-month periods ended September 30, 2020. Reference Rate Reform In March 2020, the Financial Accounting Standards Board (FASB) issued ASU No. 2020-04, “Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting,” (ASU 2020-04), to provide temporary optional guidance to ease the potential accounting burden of reference rate reform. ASU 2020-04 is effective for eligible contract modifications from January 1, 2020 through December 31, 2022. Approximately 72 percent of the Corporation’s loans at September 30, 2020 were tied to the London Interbank Offered Rate (LIBOR). The Corporation uses interest rate swaps to convert variable-LIBOR rate loans to fixed rates for approximately 10 percent of the loan portfolio. Additional contracts with exposure to LIBOR include swaps converting fixed-rate long-term debt to variable LIBOR rates. The Corporation adopted the provisions of ASU 2020-04 as of January 1, 2020 with no impact to its financial condition and results of operation at the time of adoption.

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, 2020 are not necessarily indicative of the results that may be expected for the year ending December 31, 2020. 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, 2019.

The Corporation disaggregates the loan portfolio into segments for purposes of determining the allowance for credit losses. These segments are based on the level at which the Corporation develops, documents and applies a systematic methodology to determine the allowance for credit losses. The Corporation's portfolio segments are business loans and retail loans. Business loans include the commercial, real estate construction, commercial mortgage, lease financing and international loan portfolios. Retail loans consist of residential mortgage and consumer loans, including home equity loans.

Effective January 1, 2020, the Corporation adopted the provisions of Accounting Standards Update (ASU) No. 2016-13, "Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments," commonly referred to as the current expected credit loss (CECL) model, for all financial assets, except those accounted for at fair value through net income, using the modified retrospective approach.

Under Topic 326, current expected credit losses are estimated over the contractual life of the loan portfolio, considering all available relevant information, including historical and current conditions as well as reasonable and supportable forecasts of future events. The previous accounting principle estimated probable, estimable losses inherent in the portfolio.

At adoption, the Corporation’s estimate of current expected credit losses in accordance with Topic 326 resulted in a $17 million day-one decrease in the overall allowance for credit losses, from $668 million at December 31, 2019 under the incurred loss model. The Corporation recognized a corresponding $13 million increase to retained earnings and a $4 million reduction to deferred tax assets. A similar adjustment at December 31, 2019 would have caused a 2-basis-point increase in the Common Equity Tier 1 (CET1) capital ratio. Business loans, comprising approximately 91 percent of the Corporation’s total loan portfolio at transition, consist of loans and lending arrangements with generally short contractual maturities. As a result, the allowance for credit losses for business loans decreased $42 million. The allowance for credit losses increased $25 million for retail loans, given their longer contractual maturities.

Topic 326 also requires expected credit losses on available-for-sale (AFS) debt securities to be recorded as an allowance for credit losses. For certain types of debt securities, such as U.S. Treasuries and other securities with government guarantees, entities may expect zero credit losses. The zero-loss expectation applies to all the Corporation’s securities and no allowance for credit losses was recorded on its AFS securities portfolio at transition.

The allowance for loan losses is estimated on a quarterly basis and represents management’s estimates of current expected credit losses in the Corporation’s loan portfolio. Pools of loans with similar risk characteristics are collectively evaluated while loans that no longer share risk characteristics with loan pools are evaluated individually.

Collective loss estimates are determined by applying reserve factors, designed to estimate current expected credit losses, to amortized cost balances over the remaining contractual life of the collectively evaluated portfolio. Loans with similar risk characteristics are aggregated into homogeneous pools. Business loans are assigned to pools based primarily on business line and the Corporation’s internal risk rating system. For retail loans, pools are based on loan type, past due status and credit scores. Reserve factors are based on estimated probability of default for each pool, set to a default horizon based on contractual life, and loss given default. Historical estimates are calibrated to economic forecasts over the reasonable and supportable forecast period based on the projected performance of specific economic variables that statistically correlate with each of the probability of default and loss given default pools.

The calculation of current expected credit losses is inherently subjective, as it requires management to exercise judgment in determining appropriate factors used to determine the allowance. Some of the most significant factors in the quantitative allowance estimate are assigning internal risk ratings to loans, selecting the economic forecasts used to calibrate the reserve factors and determining the reasonable and supportable forecast period.

•Internal Risk Ratings: Loss factors are dependent on loan risk ratings for business loans. Risk ratings are assigned at origination, based on inherent credit risk, and may be updated based on new information that becomes available, periodic reviews of credit quality, a change in borrower performance or modifications to lending agreements.

•Economic Forecasts: Management selects economic variables it believes to be most relevant based on the composition of the loan portfolio and customer base, including forecasted levels of employment, gross domestic product, corporate bond and treasury spreads, industrial production levels, consumer and commercial real estate price indices as well as housing statistics. Different economic forecast scenarios ranging from more benign to more severe are evaluated each reporting period to forecast losses over the contractual life of the loan portfolio.

•Forecast Period: Economic forecasts are applied over the period management believes it can estimate reasonable and supportable forecasts. Forecast periods may be adjusted in response to changes in the economic environment. To estimate losses for contractual periods that extend beyond the forecast horizon, the Corporation reverts to an average historical loss experience. The Corporation typically forecasts economic variables over a two-year horizon, followed by an immediate reversion to an average historical loss experience that generally incorporates a full economic cycle. Management reviews this methodology on at least an annual basis.

The allowance for loan losses also includes qualitative adjustments to bring the allowance to the level management believes is appropriate based on factors that have not otherwise been fully accounted for, including adjustments for foresight risk, input imprecision and model imprecision. Foresight risk reflects the inherent imprecision in forecasting economic variables, including determining the depth and duration of economic cycles and their impact to relevant economic variables. The Corporation may make qualitative adjustments based on its evaluation of different forecast scenarios and known recent events impacting relevant economic variables. Input imprecision factors address the risk that certain model inputs may not reflect all available information including (i) risk factors that have not been fully addressed in internal risk ratings, (ii) changes in lending policies and procedures, (iii) changes in the level and quality of experience held by lending management, (iv) imprecision in the risk rating system and (v) limitations in data available for certain loan portfolios. Model imprecision considers known model limitations and model updates not yet fully reflected in the quantitative estimate.

The determination of the appropriate qualitative adjustment is based on management's analysis of current and expected economic conditions and their impact to the portfolio, as well as internal credit risk movements and a qualitative assessment of the lending environment, including underwriting standards. Management recognizes the sensitivity of various assumptions made in the quantitative modeling of expected losses and may adjust reserves depending upon the level of uncertainty that currently exists in one or more assumptions.

Credit losses for loans that no longer share risk characteristics with the loan pools are estimated on an individual basis. Individual credit loss estimates are typically performed for nonaccrual loans and modified loans classified as troubled debt restructurings (TDRs) and are based on one of several methods, including the estimated fair value of the underlying collateral, observable market value of similar debt or the present value of expected cash flows. The Corporation considers certain loans to be collateral-dependent if the borrower is experiencing financial difficulty and management expects repayment for the loan to be substantially through the operation or sale of the collateral. For collateral-dependent loans, loss estimates are based on the fair value of collateral, less estimated cost to sell (if applicable). Collateral values supporting individually evaluated loans are assessed quarterly and appraisals are typically obtained at least annually.

The total allowance for loan losses is sufficient to absorb expected credit losses over the contractual life of the portfolio. Unanticipated events impacting the economy, including political instability or global events affecting the U.S. economy, could cause changes to expectations for current conditions and economic forecasts that result in an unanticipated increase in the allowance. Significant increases in current portfolio exposures or changes in credit characteristics could also increase the amount of the allowance. Such events, or others of similar nature, may result in the need for additional provision for credit losses in order to maintain an allowance that complies with credit risk and accounting policies.

The allowance for credit losses on lending-related commitments estimates current expected credit losses on collective pools of letters of credit and unused commitments to extend credit based on reserve factors, determined in a manner similar to business loans, multiplied by a probability of draw estimate, based on historical experience and credit risk, applied to commitment amounts. The allowance for credit losses on lending-related commitments is included in accrued expenses and other liabilities on the Consolidated Balance Sheets, with the corresponding charge included in the provision for credit losses on the Consolidated Statements of Comprehensive Income.

Effective January 1, 2020, the Corporation prospectively adopted the provisions of ASU No. 2017-04, “Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment,” (ASU 2017-04), eliminating the second step of goodwill impairment testing, under which the implied fair value of goodwill was determined as if the reporting unit were being acquired in a business combination. Under ASU 2017-04, the Corporation will compare the fair value of a reporting unit with its carrying amount and recognize an impairment charge for any amount by which the carrying amount exceeds the reporting unit’s fair value, to the extent that the loss recognized does not exceed the amount of goodwill allocated to that reporting unit. The adoption of ASU 2017-04 did not impact the Corporation’s current financial condition or results of operations.

Goodwill, included in accrued income and other assets on the Consolidated Balance Sheets, is initially recorded as the excess of the purchase price over the fair value of net assets acquired in a business combination and is subsequently evaluated at least annually for impairment. Goodwill impairment testing is performed at the reporting unit level, equivalent to a business segment or one level below. The Corporation has three reporting units: the Commercial Bank, the Retail Bank and Wealth Management.

The Corporation performs its annual evaluation of goodwill impairment in the third quarter of each year and may elect to perform a quantitative impairment analysis or first conduct a qualitative analysis to determine if a quantitative analysis is necessary. Additionally, the Corporation evaluates goodwill impairment on an interim basis if events or changes in circumstances between annual tests indicate additional testing may be warranted to determine if goodwill might be impaired. Factors considered in the assessment of the likelihood of impairment include macroeconomic conditions, industry and market considerations, stock performance of the Corporation and its peers, financial performance of the reporting units, and previous results of goodwill impairment tests, amongst other factors. Based on the results of the qualitative analysis, the Corporation determines whether a quantitative test is deemed necessary. The quantitative test compares the estimated fair value of identified reporting units with their carrying amount, including goodwill. If the estimated fair value of the reporting unit is less than the carrying value, an impairment charge would be recorded for the excess, not to exceed the amount of goodwill allocated to the reporting unit.

Effective January 1, 2020, the Corporation adopted the provisions of ASU No. 2018-15, “Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract," using a prospective approach for implementation costs incurred subsequent to the adoption of the standard. Amortization expense from capitalized implementation costs of hosting arrangements that are service contracts and fees associated with the hosting elements of the arrangements, are included in software expense in the Consolidated Statements of Comprehensive Income, along with other software-related expenses. For the three- and nine-month periods ended September 30, 2020, software expense included $9 million and $24 million of hosting fees that would have previously been included in outside processing fee expense.

Capitalized software, stated at cost less accumulated amortization, includes purchased software, capitalizable application development costs associated with internally developed software and cloud computing arrangements, including capitalizable implementation costs associated with hosting arrangements that are service contracts. Amortization, computed on the straight-line method, is charged to software expense in the Consolidated Statements of Comprehensive Income over the estimated useful life of the software, generally 5 years, or the term of the hosting arrangement for implementation costs related to service contracts.

In March 2020, the Financial Accounting Standards Board (FASB) issued ASU No. 2020-04, “Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting,” (ASU 2020-04), to provide temporary optional guidance to ease the potential accounting burden of reference rate reform. ASU 2020-04 is effective for eligible contract modifications from January 1, 2020 through December 31, 2022. Approximately 72 percent of the Corporation’s loans at September 30, 2020 were tied to the London Interbank Offered Rate (LIBOR). The Corporation uses interest rate swaps to convert variable-LIBOR rate loans to fixed rates for approximately 10 percent of the loan portfolio. Additional contracts with exposure to LIBOR include swaps converting fixed-rate long-term debt to variable LIBOR rates. The Corporation adopted the provisions of ASU 2020-04 as of January 1, 2020 with no impact to its financial condition and results of operation at the time of adoption.