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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 68,331	$ 57,749	$ 182,501	$ 169,887
Net income attributable to unvested awarded performance stock	(1,681)	(1,626)	(4,604)	(4,092)
Net income attributable to common shares	$ 66,650	$ 56,123	$ 177,897	$ 165,795
Weighted average number of common shares outstanding - basic	144,469	145,228	144,909	144,403
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic	(3,554)	(4,089)	(3,656)	(3,478)
Weighted average number of common shares outstanding for basic earnings per common share	140,915	141,139	141,253	140,925
Weighted average number of common shares outstanding for basic earnings per common share	140,915	141,139	141,253	140,925
Dilutive effect of stock options	2,049	1,650	1,849	1,551
Weighted average number of shares outstanding - diluted	142,964	142,789	143,102	142,476
Basic	$ 0.47	$ 0.40	$ 1.26	$ 1.18
Diluted	$ 0.47	$ 0.39	$ 1.24	$ 1.16