Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified			3 Months Ended		3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended	3 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2010 3.625% Notes Due August 1, 2020 [Member]	Dec. 31, 2011 3.625% Notes Due August 1, 2020 [Member]	Sep. 30, 2010 Floating Rate Notes Due July 30, 2010 [Member]	Dec. 31, 2012 3.875% Notes Due March 1, 2021 [Member]	Dec. 31, 2012 5.30% Notes Due March 1, 2041 [Member]	Dec. 31, 2011 5.30% Notes Due March 1, 2041 [Member]	Mar. 31, 2012 2.4% Notes Due March 21, 2022 [Member]	Dec. 31, 2012 2.4% Notes Due March 21, 2022 [Member]	Mar. 31, 2012 5.625% Notes Due February 15, 2012 [Member]	Dec. 31, 2012 Option to Increase Facility [Member]	Dec. 31, 2012 Expiring 2016 [Member]	Dec. 31, 2011 Expiring 2016 [Member]	Dec. 31, 2012 Expired 2012 [Member]	Dec. 31, 2011 Expired 2012 [Member]	Dec. 31, 2006 Dealer Remarketable Securities [Member]	Dec. 31, 2012 Dealer Remarketable Securities [Member]	Dec. 31, 2011 Dealer Remarketable Securities [Member]
Total long-term debt	$ 5,826	$ 6,045																$ 200	$ 200
Scheduled maturities of long term debt for 2013	756
Scheduled maturities of long term debt for 2014	523
Scheduled maturities of long term debt for 2015	343
Scheduled maturities of long term debt for 2016	46
Scheduled maturities of long term debt for 2017	957
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																		21
Maturity date			Aug. 01, 2020		Jul. 30, 2010	Mar. 01, 2021	Mar. 01, 2041		Mar. 01, 2022		Feb. 15, 2012						Dec. 31, 2016
Notes issued								February 2011		February 2012
Revolving credit facility												$ 500		$ 1,500		$ 500
Expiration date of revolving credit facility													October 2016		October 2012