Net Income Per Share - Reconciliation between Basic and Diluted Weighted Average Shares Outstanding (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 56,951	$ 52,415
Net income attributable to unvested awarded performance stock	(1,363)	(1,376)
Net income attributable to common shares	$ 55,588	$ 51,039
Weighted average number of common shares outstanding - basic (in shares)	142,777	145,429
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(3,417)	(3,819)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	139,360	141,610
Dilutive effect of stock options (in shares)	2,127	1,699
Weighted average number of shares outstanding - diluted (in shares)	141,487	143,309
Basic (in dollars per share)	$ 0.40	$ 0.36
Diluted (in dollars per share)	$ 0.39	$ 0.36