Repurchase of Shares of Common Stock (Detail) (USD $) In Thousands, except Share data	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Class of Stock [Line Items]
Number of Shares Repurchased and Cancelled	962,181	50,675	0	308,588	235,436	207,114	1,012,856	751,138
Amount Paid for Repurchases	$ 76,899	$ 4,302	$ 0	$ 19,329	$ 13,000	$ 11,260	$ 81,201	$ 43,589	$ 94,335
Average Price Paid per Share	$ 79.92	$ 84.90	$ 0.00	$ 62.64	$ 55.22	$ 54.36	$ 80.17	$ 58.03