Share Repurchases (Details) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Stockholders' Equity Note [Abstract]
Maximum number of shares authorized for repurchase	10,000,000	10,000,000
Shares repurchased from open market	2,530,000	3,190,000
Shares repurchased from holders of employee stock options	40,070	84,001
Total shares repurchased	2,570,070	3,274,001
Average price per share for repurchased shares (in dollars per share)	$ 59.83	$ 60.76
Number of treasury shares (in shares)	70,274,488	70,274,488	67,000,487