Regulatory Capital and Reserve Requirements Regulatory Capital and Reserve Requirements (Summary of Capital Position) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Summary of Capital Position [Line Items]
Tier 1 capital	$ 4,600	$ 4,700
Total capital	6,100	6,300
Minimum
Summary of Capital Position [Line Items]
CET 1 capital	$ 3,400	$ 3,500
CET 1 capital to risk-weighted assets	4.50%	4.50%
Tier 1 capital to risk-weighted assets - 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
Capital conservation buffer	0.025	0.025
Comerica Incorporated
Summary of Capital Position [Line Items]
CET 1 capital	$ 8,414	$ 7,884
Tier 1 capital	8,808	8,278
Total capital	10,263	9,817
Risk-weighted assets	75,901	78,871
Average assets (fourth quarter)	$ 87,538	$ 86,726
CET 1 capital to risk-weighted assets	11.09%	10.00%
Total capital to risk-weighted assets	0.1352	0.1245
Tier 1 capital to average assets	0.1006	0.0955
Capital conservation buffer	0.0552	0.0445
Tier 1 capital to risk-weighted assets	0.1160	0.1050
Comerica Bank
Summary of Capital Position [Line Items]
CET 1 capital	$ 8,007	$ 7,801
Tier 1 capital	8,007	7,801
Total capital	9,362	9,190
Risk-weighted assets	75,783	78,781
Average assets (fourth quarter)	$ 87,423	$ 86,608
CET 1 capital to risk-weighted assets	10.57%	9.90%
Total capital to risk-weighted assets	0.1235	0.1167
Tier 1 capital to average assets	0.0916	0.0901
Capital conservation buffer	0.0435	0.0367
Tier 1 capital to risk-weighted assets	0.1057	0.0990