Repurchase of Shares of Common Stock (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended						9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2012	Sep. 30, 2011
Class of Stock [Line Items]
Number of Shares Repurchased and Cancelled	43,500	0	38,552	962,181	50,675	0	82,052	1,012,856
Amount Paid for Repurchases	$ 3,913	$ 0	$ 2,945	$ 76,899	$ 4,302	$ 0	$ 2,945	$ 81,201
Average Price Paid per Share	$ 89.97	$ 0.00	$ 76.38	$ 79.92	$ 84.90	$ 0.00	$ 83.58	$ 80.17