Regulatory Capital and Reserve Requirements Regulatory Capital and Reserve Requirements (Summary of Capital Position) (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Comerica Incorporated
Summary of Capital Position [Line Items]
CET 1 capital	$ 6,919	$ 7,470
Tier 1 capital	6,919	7,470
Total capital	8,282	8,855
Risk-weighted assets	68,273	67,047
Average assets (fourth quarter)	$ 72,773	$ 71,070
CET 1 capital to risk-weighted assets	10.13%	11.14%
Tier 1 capital to risk-weighted assets	10.13%	11.14%
Total capital to risk-weighted assets	12.13%	13.21%
Tier 1 capital to average assets	9.51%	10.51%
Capital conservation buffer	4.13%	5.14%
CET 1 capital - minimum (Consolidated)	$ 3,100	$ 3,000
Tier 1 capital - minimum (Consolidated)	4,100	4,000
Total capital - minimum (Consolidated)	$ 5,500	$ 5,400
CET 1 capital to risk-weighted assets - minimum	4.50%	4.50%
Tier 1 capital to risk-weighted assets - minimum	6.00%	6.00%
Total capital to risk-weighted assets - minimum	8.00%	8.00%
Tier 1 capital to average assets - minimum	4.00%	4.00%
Comerica Bank
Summary of Capital Position [Line Items]
CET 1 capital	$ 7,199	$ 7,229
Tier 1 capital	7,199	7,229
Total capital	8,371	8,433
Risk-weighted assets	68,071	66,857
Average assets (fourth quarter)	$ 72,564	$ 70,905
CET 1 capital to risk-weighted assets	10.58%	10.81%
Tier 1 capital to risk-weighted assets	10.58%	10.81%
Total capital to risk-weighted assets	12.30%	12.61%
Tier 1 capital to average assets	9.92%	10.20%
Capital conservation buffer	4.30%	4.61%
CET 1 capital to risk-weighted assets - minimum	4.50%	4.50%
Tier 1 capital to risk-weighted assets - minimum	6.00%	6.00%
Total capital to risk-weighted assets - minimum	8.00%	8.00%
Tier 1 capital to average assets - minimum	4.00%	4.00%

Average total assets for leverage ratio as defined in the regulations.

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The amount of capital in excess of regulatory required minimum capital ratios.

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The minimum Common Equity Tier One Risk Based capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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Common Equity Tier One Risk Based Capital To Risk Weighted Assets as defined by regulations.

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The minimum Common Equity Tier One Risked Based Capital (CET 1 divided by riske weighted assets) required for capital adequacy under the regulatory framework for prompt corrective action.

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Amount of risk-weighted assets as calculated in accordance with U.S. Federal regulatory capital standards.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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