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								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 57,935	$ 67,427	$ 61,005	$ 56,951	$ 24,395	$ 68,331	$ 61,755	$ 52,415	$ 243,318	$ 206,896	$ 217,112
Net income attributable to unvested awarded performance stock									(5,695)	(5,186)	(5,446)
Net income attributable to common shares									$ 237,623	$ 201,710	$ 211,666
Weighted average number of common shares outstanding - basic (in shares)									141,113	144,568	144,662
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)									(3,303)	(3,624)	(3,629)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)									137,810	140,944	141,033
Dilutive effect of stock options (in shares)									2,302	1,947	1,591
Weighted average number of shares outstanding - diluted (in shares)									140,112	142,891	142,624
Basic (in dollars per share)	$ 0.0041	$ 0.0048	$ 0.0043	$ 0.0040	$ 0.0017	$ 0.0047	$ 0.0043	$ 0.0036	$ 1.72	$ 1.43	$ 1.50
Diluted (in dollars per share)	$ 0.0041	$ 0.0047	$ 0.0043	$ 0.0039	$ 0.0017	$ 0.0047	$ 0.0042	$ 0.0036	$ 1.70	$ 1.41	$ 1.48