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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 187,505	$ 75,913	$ 66,102	$ 70,110	$ 57,626	$ 71,545	$ 66,250	$ 62,070	$ 399,630	$ 257,491	$ 243,318
Net income attributable to unvested awarded performance stock									(9,746)	(6,705)	(5,695)
Net income attributable to common shares									$ 389,884	$ 250,786	$ 237,623
Weighted average number of common shares outstanding - basic (in shares)									139,697	139,779	141,113
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)									(3,407)	(3,640)	(3,303)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)									136,290	136,139	137,810
Dilutive effect of stock options (in shares)									2,503	1,665	2,302
Weighted average number of shares outstanding - diluted (in shares)									138,793	137,804	140,112
Basic (in dollars per share)	$ 0.0135	$ 0.0054	$ 0.0047	$ 0.0050	$ 0.0041	$ 0.0051	$ 0.0047	$ 0.0045	$ 2.86	$ 1.84	$ 1.72
Diluted (in dollars per share)	$ 0.0132	$ 0.0053	$ 0.0046	$ 0.0049	$ 0.0041	$ 0.0050	$ 0.0047	$ 0.0044	$ 2.81	$ 1.82	$ 1.70