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Stock-based Compensation - Fair Value of Stock Appreciation Rights Estimated, Black-Scholes Option Pricing Model (Detail) - Performance-Based Stock Appreciation Rights [Member] - USD ($)
9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate, Minimum 1.35% 1.81%
Risk-free interest rate, Maximum 1.82% 1.94%
Expected life (years) 6 years 6 years
Estimated volatility factor, Minimum 62.74% 66.07%
Estimated volatility factor, Maximum 64.63% 67.34%
Expected dividends $ 0 $ 0