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Share-Based Compensation Share-Based Compensation (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of shares originally available for grant	19,400,000
Shares available for grant	9,500,000
Total intrinsic value of stock options exercised	$ 104	$ 46	$ 12
Shares in treasury	55,306,483	52,851,156
Restricted Stock
Total fair value of restricted stock awards that fully vested	$ 19	$ 22	18
Restricted Stock \| Minimum
Share-based compensation vesting period	3 years
Restricted Stock \| Maximum
Share-based compensation vesting period	5 years
Restricted Stock Units (RSUs)
Total fair value of restricted stock awards that fully vested	$ 10	$ 11	$ 0
Restricted Stock Units (RSUs) \| Minimum
Share-based compensation vesting period	1 year
Restricted Stock Units (RSUs) \| Maximum
Share-based compensation vesting period	3 years
Stock Options \| Minimum
Share-based compensation vesting period	1 year
Stock Options \| Maximum
Share-based compensation vesting period	4 years
Comerica Incorporated
Closing stock price per share	$ 86.81