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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 61,755	$ 52,007	$ 114,170	$ 112,138
Net income attributable to unvested awarded performance stock	(1,531)	(1,157)	(2,915)	(2,467)
Net income attributable to common shares	$ 60,224	$ 50,850	$ 111,255	$ 109,671
Weighted average number of common shares outstanding - basic	144,840	144,041	145,133	143,984
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic	(3,590)	(3,205)	(3,705)	(3,168)
Weighted average number of common shares outstanding for basic earnings per common share	141,250	140,836	141,428	140,816
Weighted average number of common shares outstanding for basic earnings per common share	141,250	140,836	141,428	140,816
Dilutive effect of stock options	1,782	2,185	1,741	2,122
Weighted average number of shares outstanding - diluted	143,032	143,021	143,169	142,938
Basic	$ 0.43	$ 0.36	$ 0.79	$ 0.78
Diluted	$ 0.42	$ 0.36	$ 0.78	$ 0.77