Assumptions used in Valuation of Performance Awards Granted (Detail) (Performance-Based Restricted Stock, USD $)	12 Months Ended
Dec. 31, 2011 Year	Dec. 31, 2010 Year	Dec. 31, 2009 Year
Assumptions
Expected volatility	51.00%	51.00%	48.00%
Risk-free interest rate	1.04%	1.41%	1.37%
Stock beta	1.006	1.008	0.973
Expected life (in years)	3.00	3.00	3.00
Minimum
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Fair value per share at grant date	38.94	42.38	41.18
Maximum
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Fair value per share at grant date	64.89	50.95	44.94

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 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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- Definition

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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