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
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 90,828	$ 70,110
Net income attributable to unvested awarded performance stock	(1,902)	(1,686)
Net income attributable to common shares	$ 88,926	$ 68,424
Weighted average number of common shares outstanding - basic (in shares)	275,952	280,224
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(5,780)	(6,737)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	270,172	273,487
Dilutive effect of stock options (in shares)	5,542	4,565
Weighted average number of shares outstanding - diluted (in shares)	275,714	278,052
Basic (in dollars per share)	$ 0.33	$ 0.25
Diluted (in dollars per share)	$ 0.32	$ 0.25