Contingent Liabilities	9 Months Ended
Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Contingent Liabilities	CONTINGENT LIABILITIES Legal Proceedings As previously reported in the Corporation's Form 10-K for the year ended December 31, 2017 and Forms 10-Q for the quarters ended March 31, 2018 and June 30, 2018, Comerica Bank, a wholly owned subsidiary of the Corporation, was named in November 2011 as a third-party defendant in Butte Local Development v. Masters Group v. Comerica Bank (“the case”), for lender liability. The case was tried in January 2014, in the Montana Second District Judicial Court for Silver Bow County in Butte, Montana. On January 17, 2014, a jury awarded Masters $52 million against the Bank. On July 1, 2015, after an appeal filed by the Corporation, the Montana Supreme Court reversed the judgment against the Corporation and remanded the case for a new trial with instructions that Michigan contract law should apply and dismissing all other claims. The case was retried in the same district court, without a jury, in January 2017, and the Corporation awaits a ruling. Management believes current reserves related to this case are adequate in the event of a negative outcome. The Corporation and certain of its subsidiaries are subject to various other pending or threatened legal proceedings arising out of the normal course of business or operations. The Corporation believes it has meritorious defenses to the claims asserted against it in its other currently outstanding legal proceedings and, with respect to such legal proceedings, intends to continue to defend itself vigorously, litigating or settling cases according to management’s judgment as to what is in the best interests of the Corporation and its shareholders. Settlement may result from the Corporation's determination that it may be more prudent financially to settle, rather than litigate, and should not be regarded as an admission of liability. On at least a quarterly basis, the Corporation assesses its potential liabilities and contingencies in connection with outstanding legal proceedings utilizing the latest information available. On a case-by-case basis, reserves are established for those legal claims for which it is probable that a loss will be incurred either as a result of a settlement or judgment, and the amount of such loss can be reasonably estimated. The actual costs of resolving these claims may be substantially higher or lower than the amounts reserved. Based on current knowledge, and after consultation with legal counsel, management believes current reserves are adequate, and the amount of any incremental liability arising from these matters is not expected to have a material adverse effect on the Corporation’s consolidated financial condition, results of operations or cash flows. Legal fees of $4 million were included in other noninterest expenses for both the three-month periods ended September 30, 2018 and 2017 and $11 million and $14 million for the nine-month periods ended September 30, 2018 and 2017, respectively. For matters where a loss is not probable, the Corporation has not established legal reserves. The Corporation believes the estimate of the aggregate range of reasonably possible losses, in excess of reserves established, for all legal proceedings in which it is involved is from zero to approximately $33 million at September 30, 2018. This estimated aggregate range of reasonably possible losses is based upon currently available information for those proceedings in which the Corporation is involved, taking into account the Corporation’s best estimate of such losses for those cases for which such estimate can be made. For certain cases, the Corporation does not believe an estimate can currently be made. The Corporation’s estimate involves significant judgment, given the varying stages of the proceedings (including the fact many are currently in preliminary stages), the existence in certain proceedings of multiple defendants (including the Corporation) whose share of liability has yet to be determined, the numerous yet-unresolved issues in many of the proceedings (including issues regarding class certification and the scope of many of the claims) and the attendant uncertainty of the various potential outcomes of such proceedings. Accordingly, the Corporation’s estimate will change from time to time, and actual losses may be more or less than the current estimate. In the event of unexpected future developments, it is possible the ultimate resolution of these matters, if unfavorable, may be material to the Corporation's consolidated financial condition, results of operations or cash flows. For information regarding income tax contingencies, refer to note 11.

CONTINGENT LIABILITIES

Legal Proceedings

As previously reported in the Corporation's Form 10-K for the year ended December 31, 2017 and Forms 10-Q for the quarters ended March 31, 2018 and June 30, 2018, Comerica Bank, a wholly owned subsidiary of the Corporation, was named in November 2011 as a third-party defendant in Butte Local Development v. Masters Group v. Comerica Bank (“the case”), for lender liability. The case was tried in January 2014, in the Montana Second District Judicial Court for Silver Bow County in Butte, Montana. On January 17, 2014, a jury awarded Masters $52 million against the Bank. On July 1, 2015, after an appeal filed by the Corporation, the Montana Supreme Court reversed the judgment against the Corporation and remanded the case for a new trial with instructions that Michigan contract law should apply and dismissing all other claims. The case was retried in the same district court, without a jury, in January 2017, and the Corporation awaits a ruling. Management believes current reserves related to this case are adequate in the event of a negative outcome.

The Corporation and certain of its subsidiaries are subject to various other pending or threatened legal proceedings arising out of the normal course of business or operations. The Corporation believes it has meritorious defenses to the claims asserted against it in its other currently outstanding legal proceedings and, with respect to such legal proceedings, intends to continue to defend itself vigorously, litigating or settling cases according to management’s judgment as to what is in the best interests of the Corporation and its shareholders. Settlement may result from the Corporation's determination that it may be more prudent financially to settle, rather than litigate, and should not be regarded as an admission of liability. On at least a quarterly basis, the Corporation assesses its potential liabilities and contingencies in connection with outstanding legal proceedings utilizing the latest information available. On a case-by-case basis, reserves are established for those legal claims for which it is probable that a loss will be incurred either as a result of a settlement or judgment, and the amount of such loss can be reasonably estimated. The actual costs of resolving these claims may be substantially higher or lower than the amounts reserved. Based on current knowledge, and after consultation with legal counsel, management believes current reserves are adequate, and the amount of any incremental liability arising from these matters is not expected to have a material adverse effect on the Corporation’s consolidated financial condition, results of operations or cash flows. Legal fees of $4 million were included in other noninterest expenses for both the three-month periods ended September 30, 2018 and 2017 and $11 million and $14 million for the nine-month periods ended September 30, 2018 and 2017, respectively.

For matters where a loss is not probable, the Corporation has not established legal reserves. The Corporation believes the estimate of the aggregate range of reasonably possible losses, in excess of reserves established, for all legal proceedings in which it is involved is from zero to approximately $33 million at September 30, 2018. This estimated aggregate range of reasonably possible losses is based upon currently available information for those proceedings in which the Corporation is involved, taking into account the Corporation’s best estimate of such losses for those cases for which such estimate can be made. For certain cases, the Corporation does not believe an estimate can currently be made. The Corporation’s estimate involves significant judgment, given the varying stages of the proceedings (including the fact many are currently in preliminary stages), the existence in certain proceedings of multiple defendants (including the Corporation) whose share of liability has yet to be determined, the numerous yet-unresolved issues in many of the proceedings (including issues regarding class certification and the scope of many of the claims) and the attendant uncertainty of the various potential outcomes of such proceedings. Accordingly, the Corporation’s estimate will change from time to time, and actual losses may be more or less than the current estimate.

In the event of unexpected future developments, it is possible the ultimate resolution of these matters, if unfavorable, may be material to the Corporation's consolidated financial condition, results of operations or cash flows.

For information regarding income tax contingencies, refer to note 11.