Stockholders' Equity - Weighted-Average Assumptions Used in the Monte Carlo Simulation (Detail) (Market Performance Based Restricted Stock Units, USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Market Performance Based Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	3 years	3 years
Expected volatility	54.00%	58.40%
Risk-free interest rate	0.40%	1.30%
Expected dividends	0.00%	0.00%
Weighted average fair value per share at grant date (usd per share)	$ 29.45	$ 22.12

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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