Background and Basis of Presentation - Summary of Earnings, Weighted Average Common Shares and Potential Common Shares Outstanding (Detail) (USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Numerator:    
Net income for basic and diluted earnings per share $ 39,476 $ 64,487
Denominator:    
Weighted average shares outstanding - basic 188,480 185,942
Dilutive effect of equity-based awards 4,657 4,980
Weighted average common shares outstanding - diluted 193,137 190,922
Earnings per share:    
Basic $ 0.21 $ 0.35
Diluted $ 0.20 $ 0.34