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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income	$ 66,250	$ 61,005	$ 128,320	$ 117,956
Net income attributable to unvested awarded performance stock	(1,896)	(1,439)	(3,337)	(2,802)
Net income attributable to common shares	$ 64,354	$ 59,566	$ 124,983	$ 115,154
Weighted average number of common shares outstanding - basic (in shares)	139,998	140,839	139,395	141,803
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(4,006)	(3,321)	(3,625)	(3,369)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	135,992	137,518	135,770	138,434
Dilutive effect of stock options (in shares)	1,589	2,310	1,490	2,213
Weighted average number of shares outstanding - diluted (in shares)	137,581	139,828	137,260	140,647
Basic (in dollars per share)	$ 0.47	$ 0.43	$ 0.92	$ 0.83
Diluted (in dollars per share)	$ 0.47	$ 0.43	$ 0.91	$ 0.82