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Regulatory Capital and Reserve Requirements Regulatory Capital and Reserve Requirements (Summary of Capital Position) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Summary of Capital Position [Line Items]      
Tier 1 capital $ 4,400 $ 4,600  
Total capital 5,800   $ 6,100
Minimum      
Summary of Capital Position [Line Items]      
CET 1 capital $ 3,300 $ 3,400  
CET 1 capital to risk-weighted assets 4.50% 4.50%  
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Capital Adequacy, Minimum 0.060 0.060  
Total capital to risk-weighted assets 0.080 0.080  
Tier 1 capital to average assets 0.040 0.040  
Banking Regulation, Capital Conservation Buffer, Total Risk-Based Capital, Actual   0.025  
Capital conservation buffer 0.025    
Comerica Incorporated      
Summary of Capital Position [Line Items]      
CET 1 capital $ 8,667 $ 8,414  
Tier 1 capital 9,061 8,808  
Total capital 10,363 10,263  
Risk-weighted assets 72,903 75,901  
Average assets (fourth quarter) $ 81,797 $ 87,538  
CET 1 capital to risk-weighted assets 11.89% 11.09%  
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Actual 0.1243 0.1160  
Total capital to risk-weighted assets 0.1421 0.1352  
Tier 1 capital to average assets 0.1108 0.1006  
Capital conservation buffer 0.0621 0.0552  
Comerica Bank      
Summary of Capital Position [Line Items]      
CET 1 capital $ 8,547 $ 8,007  
Tier 1 capital 8,547 8,007  
Total capital 9,799 9,362  
Risk-weighted assets 72,833 75,783  
Average assets (fourth quarter) $ 81,643 $ 87,423  
CET 1 capital to risk-weighted assets 11.74% 10.57%  
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Actual 0.1174 0.1057  
Total capital to risk-weighted assets 0.1345 0.1235  
Tier 1 capital to average assets 0.1047 0.0916  
Capital conservation buffer 0.0545 0.0435