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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 ($)
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate, Minimum 1.35% 1.81%
Expected life (years) 6 years 6 years
Risk-free interest rate, Maximum 1.82% 1.94%
Expected dividends $ 0 $ 0
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated volatility factor 64.63% 67.34%
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated volatility factor 64.04% 66.59%