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Share-Based Compensation (Estimated Weighted-Average Grant-Date Fair Value Per Option Share and the Underlying Binomial Option-Pricing Model Assumptions) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation [Abstract]
Weighted-average grant-date fair value per option	$ 8.63	$ 11.58	$ 11.07
Risk-free interest rates	2.16%	3.43%	3.73%
Expected dividend yield	3.00%	3.00%	3.00%
Expected volatility factors of the market price of Comerica common stock	39.00%	38.00%	40.00%
Expected option life (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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