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, 2013	Mar. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 60,131	$ 49,433
Net income attributable to unvested awarded performance stock	(1,308)	(1,499)
Net income attributable to common shares	$ 58,823	$ 47,934
Weighted average number of common shares outstanding - basic	143,926	143,347
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic	(3,130)	(4,346)
Weighted average number of common shares outstanding for basic earnings per common share	140,796	139,001
Dilutive effect of stock options	2,151	2,499
Weighted average number of shares outstanding - diluted	142,947	141,500
Basic	$ 0.42	$ 0.34
Diluted	$ 0.41	$ 0.34