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, 2011	Dec. 31, 2010	Dec. 31, 2009
Weighted-average grant-date fair value per option	$ 11.58	$ 11.07	$ 6.55
Risk-free interest rates	3.43%	3.73%	3.08%
Expected dividend yield	3.00%	3.00%	4.62%
Expected volatility factors of the market price of Comerica common stock	38.00%	40.00%	58.00%
Expected option life (in years)	6.1	6.1	6.4

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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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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