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, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 52,415	$ 60,131
Net income attributable to unvested awarded performance stock	(1,376)	(1,308)
Net income attributable to common shares	$ 51,039	$ 58,823
Weighted average number of common shares outstanding - basic	145,429	143,926
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic	(3,819)	(3,130)
Weighted average number of common shares outstanding for basic earnings per common share	141,610	140,796
Dilutive effect of stock options	1,699	2,151
Weighted average number of shares outstanding - diluted	143,309	142,947
Basic	$ 0.36	$ 0.42
Diluted	$ 0.36	$ 0.41