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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 75,913	$ 71,545	$ 212,125	$ 199,865
Net income attributable to unvested awarded performance stock	(1,852)	(1,873)	(5,181)	(5,210)
Net income attributable to common shares	$ 74,061	$ 69,672	$ 206,944	$ 194,655
Weighted average number of common shares outstanding - basic (in shares)	139,756	140,129	140,012	139,642
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(3,410)	(3,668)	(3,420)	(3,640)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	136,346	136,461	136,592	136,002
Dilutive effect of stock options (in shares)	2,547	1,721	2,419	1,582
Weighted average number of shares outstanding - diluted (in shares)	138,893	138,182	139,011	137,584
Basic (in dollars per share)	$ 0.54	$ 0.51	$ 1.52	$ 1.43
Diluted (in dollars per share)	$ 0.53	$ 0.50	$ 1.49	$ 1.41