Stock Repurchase Plans (Details) (USD $)	0 Months Ended			1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 19, 2013	Dec. 11, 2012	Jan. 03, 2012	Feb. 24, 2011	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Stock repurchase program
Amount of shares authorized to be repurchased under Stock Repurchase Program		$ 500,000,000	$ 250,000,000	$ 60,000,000
Period over which shares are authorized to be repurchased		3 years	4 years	3 years
Total number of shares purchased	5,811,000						4,249,000	1,617,000
Value of shares repurchased	$ 385,600,000				$ 375,290,000	$ 44,457,000	$ 232,000,000	$ 60,000,000
Average price paid per share (in dollars per share)	$ 66.36