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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 57,626	$ 71,545	$ 66,250	$ 62,070	$ 57,935	$ 67,427	$ 61,005	$ 56,951	$ 257,491	$ 243,318	$ 206,896
Net income attributable to unvested awarded performance stock									(6,705)	(5,695)	(5,186)
Net income attributable to common shares									$ 250,786	$ 237,623	$ 201,710
Weighted average number of common shares outstanding - basic (in shares)									139,779	141,113	144,568
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)									(3,640)	(3,303)	(3,624)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)									136,139	137,810	140,944
Dilutive effect of stock options (in shares)									1,665	2,302	1,947
Weighted average number of shares outstanding - diluted (in shares)									137,804	140,112	142,891
Basic (in dollars per share)	$ 0.0041	$ 0.0051	$ 0.0047	$ 0.0045	$ 0.0041	$ 0.0048	$ 0.0043	$ 0.0040	$ 1.84	$ 1.72	$ 1.43
Diluted (in dollars per share)	$ 0.0041	$ 0.0050	$ 0.0047	$ 0.0044	$ 0.0041	$ 0.0047	$ 0.0043	$ 0.0039	$ 1.82	$ 1.70	$ 1.41