Share Repurchases (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Share Repurchases [Abstract]
Maximum number of shares Board of Directors authorized for repurchase (in shares)	10,000,000
Authorized repurchase expiration	May 2012
Number of shares repurchased during period (in shares)	660,000
Shares repurchased from holders of employee stock options (in shares)	43,931
Total shares repurchased (in shares)	703,931
Average price per share for repurchased shares (in dollars per share)	$ 64.15
Number of treasury shares (in shares)	67,704,418	67,000,487