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Regulatory Capital and Reserve Requirements Regulatory Capital and Reserve Requirements (Summary of Capital Position) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020
Summary of Capital Position [Line Items] |  |
Tier 1 capital | $ 4,200 | $ 4,000
Total capital | 5,600 | 5,400
Minimum |  |
Summary of Capital Position [Line Items] |  |
CET 1 capital | $ 3,100 | $ 3,000
CET1 capital to risk-weighted assets | 4.50% | 4.50%
Tier1 capital to risk-weighted assets | 6.00% | 6.00%
Total capital to risk-weighted assets | 8.00% | 8.00%
Leverage ratio | 4.00% | 4.00%
Conservation buffer | 2.50% | 2.50%
Comerica Incorporated |  |
Summary of Capital Position [Line Items] |  |
CET 1 capital | $ 7,064 | $ 6,919
Tier 1 capital | 7,458 | 7,313
Total capital | 8,608 | 8,833
Risk-weighted assets | 69,708 | 66,931
Average assets (fourth quarter) | $ 96,417 | $ 84,705
CET1 capital to risk-weighted assets | 10.13% | 10.34%
Tier1 capital to risk-weighted assets | 10.70% | 10.93%
Total capital to risk-weighted assets | 12.35% | 13.20%
Leverage ratio | 7.74% | 8.63%
Conservation buffer | 4.35% | 4.93%
Comerica Bank |  |
Summary of Capital Position [Line Items] |  |
CET 1 capital | $ 7,634 | $ 7,278
Tier 1 capital | 7,634 | 7,278
Total capital | 8,584 | 8,547
Risk-weighted assets | 69,542 | 66,759
Average assets (fourth quarter) | $ 96,216 | $ 84,536
CET1 capital to risk-weighted assets | 10.98% | 10.90%
Tier1 capital to risk-weighted assets | 10.98% | 10.90%
Total capital to risk-weighted assets | 12.34% | 12.80%
Leverage ratio | 7.93% | 8.61%
Conservation buffer | 4.34% | 4.80%

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- Definition Amount of total capital as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
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