Net Income Per Share - Reconciliation between Basic and Diluted Weighted Average Shares Outstanding (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 70,110	$ 62,070
Net income attributable to unvested awarded performance stock	(1,685)	(1,451)
Net income attributable to common shares	$ 68,425	$ 60,619
Weighted average number of common shares outstanding - basic (in shares)	140,112	138,793
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(3,368)	(3,245)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	136,744	135,548
Dilutive effect of stock options (in shares)	2,282	1,392
Weighted average number of shares outstanding - diluted (in shares)	139,026	136,940
Basic (in dollars per share)	$ 0.50	$ 0.45
Diluted (in dollars per share)	$ 0.49	$ 0.44