Share Repurchases (Detail) (USD $)	3 Months Ended									12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Equity, Class of Treasury Stock [Line Items]
Total Number of Shares Purchased	376,911	749,076	597,026	256,094	35,566	97,115	558,003	573,600	141,576	1,979,107	1,264,284	141,576
Average Cost Paid Per Share	$ 45.61	$ 43.03	$ 47.55	$ 50.96	$ 48.04	$ 42.25	$ 40.32	$ 41.27	$ 47.72	$ 45.91	$ 41.11	$ 47.72