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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income	$ 71,545	$ 67,427	$ 199,865	$ 185,383
Net income attributable to unvested awarded performance stock	(1,873)	(1,565)	(5,210)	(4,370)
Net income attributable to common shares	$ 69,672	$ 65,862	$ 194,655	$ 181,013
Weighted average number of common shares outstanding - basic (in shares)	140,129	140,955	139,642	141,517
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(3,668)	(3,271)	(3,640)	(3,336)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	136,461	137,684	136,002	138,181
Dilutive effect of stock options (in shares)	1,721	2,357	1,582	2,265
Weighted average number of shares outstanding - diluted (in shares)	138,182	140,041	137,584	140,446
Basic (in dollars per share)	$ 0.51	$ 0.48	$ 1.43	$ 1.31
Diluted (in dollars per share)	$ 0.50	$ 0.47	$ 1.41	$ 1.29