Assumptions used in Valuation Performance Awards Granted (Detail) (Performance Based Restricted Stock, USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assumptions
Expected volatility	37.00%	51.00%	51.00%
Risk-free interest rate	0.30%	1.04%	1.41%
Stock beta	0.913	1.006	1.008
Expected life (in years)	3 years	3 years	3 years
Minimum
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Fair value per share at grant date	54.90	38.94	42.38
Maximum
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Fair value per share at grant date	63.64	64.89	50.95

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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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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