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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 66,102	$ 66,250	$ 136,212	$ 128,320
Net income attributable to unvested awarded performance stock	(1,642)	(1,896)	(3,329)	(3,337)
Net income attributable to common shares	$ 64,460	$ 64,354	$ 132,883	$ 124,983
Weighted average number of common shares outstanding - basic (in shares)	140,173	139,998	140,143	139,395
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(3,481)	(4,006)	(3,425)	(3,625)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	136,692	135,992	136,718	135,770
Dilutive effect of stock options (in shares)	2,409	1,589	2,346	1,490
Weighted average number of shares outstanding - diluted (in shares)	139,101	137,581	139,064	137,260
Basic (in dollars per share)	$ 0.47	$ 0.47	$ 0.97	$ 0.92
Diluted (in dollars per share)	$ 0.46	$ 0.47	$ 0.96	$ 0.91