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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 73,922	$ 66,102	$ 164,750	$ 136,212
Net income attributable to unvested awarded performance stock	(1,618)	(1,642)	(3,529)	(3,329)
Net income attributable to common shares	$ 72,304	$ 64,460	$ 161,221	$ 132,883
Weighted average number of common shares outstanding - basic (in shares)	276,123	280,346	276,038	280,286
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(6,042)	(6,962)	(5,912)	(6,850)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	270,081	273,384	270,126	273,436
Dilutive effect of stock options (in shares)	5,827	4,818	5,683	4,692
Weighted average number of shares outstanding - diluted (in shares)	275,908	278,202	275,809	278,128
Basic (in dollars per share)	$ 0.27	$ 0.24	$ 0.60	$ 0.49
Diluted (in dollars per share)	$ 0.26	$ 0.23	$ 0.58	$ 0.48