Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended		3 Months Ended	12 Months Ended		3 Months Ended			12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2010 3.625% Notes Due August 1, 2020 [Member]	Dec. 31, 2010 3.625% Notes Due August 1, 2020 [Member]	Sep. 30, 2010 Floating Rate Notes Due July 30, 2010 [Member]	Dec. 31, 2011 3.875% Notes Due March 1, 2021 [Member]	Dec. 31, 2011 5.30% Notes Due March 1, 2041 [Member]	Mar. 31, 2012 2.4% Notes Due March 21, 2022 [Member]	Mar. 31, 2012 5.625% Notes Due February 15, 2012 [Member]	Dec. 31, 2011 Option to Increase Facility [Member]	Dec. 31, 2011 Expiring 2016 [Member]	Dec. 31, 2011 Expiring 2012 [Member]	Dec. 31, 2006 Dealer Remarketable Securities [Member]	Dec. 31, 2011 Dealer Remarketable Securities [Member]	Dec. 31, 2010 Dealer Remarketable Securities [Member]
Total long-term debt		$ 6,045	$ 5,385												$ 200	$ 200
Scheduled maturities of long term debt for 2012		619
Scheduled maturities of long term debt for 2013		592
Scheduled maturities of long term debt for 2014		524
Scheduled maturities of long term debt for 2015		344
Scheduled maturities of long term debt for 2016		51
Face amount of note					250		250	450	300	400				200
Interest rate of note				3.625%			3.875%	5.30%	2.40%	5.625%
Fair value of remarketable securities option															22.9
Maturity date	Feb. 15, 2012			Aug. 01, 2020		Jul. 30, 2010	Mar. 01, 2021	Mar. 01, 2041	Mar. 01, 2022					Dec. 31, 2016
Notes issued	February 9, 2012
Revolving credit facility			1,330								500	1,500	500
Expiration date of revolving credit facility												October 2016	October 2012
Amount borrowed under revolving credit facility		$ 0