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Summary of Assumptions Used to Estimate Fair Value (Detail) (Stock Appreciation Rights (SARs))	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	47.10%	44.30%
Weighted-average volatility	47.10%	44.30%
Expected dividend rate	0.00%	0.00%
Expected term (in years)	4 years 8 months 12 days	4 years 7 months 6 days
Risk-free rate	0.80%	2.00%

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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