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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 73,452	$ 106,053	$ 73,922	$ 90,828	$ 187,505	$ 75,913	$ 66,102	$ 70,110	$ 344,255	$ 399,630	$ 257,491
Net income attributable to unvested awarded performance stock									(8,297)	(9,746)	(6,705)
Net income attributable to common shares									$ 335,958	$ 389,884	$ 250,786
Weighted average number of common shares outstanding - basic (in shares)									277,663	279,394	279,558
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)									(6,692)	(6,814)	(7,280)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)									270,971	272,580	272,278
Dilutive effect of stock options (in shares)									4,550	5,006	3,330
Weighted average number of shares outstanding - diluted (in shares)									275,521	277,586	275,608
Basic (in dollars per share)	$ 0.0026	$ 0.0038	$ 0.0027	$ 0.0033	$ 0.0068	$ 0.0027	$ 0.0024	$ 0.0025	$ 1.24	$ 1.43	$ 0.92
Diluted (in dollars per share)	$ 0.0026	$ 0.0038	$ 0.0026	$ 0.0032	$ 0.0066	$ 0.0027	$ 0.0023	$ 0.0025	$ 1.22	$ 1.40	$ 0.91