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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 61,005	$ 61,755	$ 117,956	$ 114,170
Net income attributable to unvested awarded performance stock	(1,439)	(1,531)	(2,802)	(2,915)
Net income attributable to common shares	$ 59,566	$ 60,224	$ 115,154	$ 111,255
Weighted average number of common shares outstanding - basic (in shares)	140,839	144,840	141,803	145,133
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(3,321)	(3,590)	(3,369)	(3,705)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	137,518	141,250	138,434	141,428
Dilutive effect of stock options (in shares)	2,310	1,782	2,213	1,741
Weighted average number of shares outstanding - diluted (in shares)	139,828	143,032	140,647	143,169
Basic (in dollars per share)	$ 0.43	$ 0.43	$ 0.83	$ 0.79
Diluted (in dollars per share)	$ 0.43	$ 0.42	$ 0.82	$ 0.78