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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net income	$ 57,749	$ 49,504	$ 169,887	$ 141,408
Net income attributable to unvested awarded performance stock	(1,626)	(1,428)	(4,092)	(4,174)
Net income attributable to common shares	$ 56,123	$ 48,076	$ 165,795	$ 137,234
Weighted average number of common shares outstanding - basic	145,228	143,607	144,403	143,418
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic	(4,089)	(4,142)	(3,478)	(4,233)
Weighted average number of common shares outstanding for basic earnings per common share	141,139	139,465	140,925	139,185
Dilutive effect of stock options	1,650	2,632	1,551	2,584
Weighted average number of shares outstanding - diluted	142,789	142,097	142,476	141,769
Basic	$ 0.40	$ 0.34	$ 1.18	$ 0.99
Diluted	$ 0.39	$ 0.34	$ 1.16	$ 0.97