Long-Term Debt (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009 $350 Million Note due 2014 at 3.70%	Dec. 31, 2009 $500 Million Note due 2019 at 5.05%	Jul. 01, 2009 $500 Million Note due 2019 at 5.05%	Dec. 31, 2010 $500 Million Note due 2015 at 1.85%	Dec. 31, 2010 $700 Million Note due 2021 at 3.50%	Dec. 31, 2010 $300 Million Note due 2040 at 5.05%	Dec. 31, 2010 $1.919 Billion NGSC debt securities due 2016 to 2036, rates from 6.98% to 7.875%	Dec. 31, 2011 First Credit Facility	Dec. 31, 2011 Second Credit Facility	Dec. 31, 2011 Terminated Credit Facility
Long-term debt maturities (Table Amounts) [Abstract]
2012		$ 5
2013		5
2014		353
2015		502
2016		109
Thereafter		2,955
Total principal payments		3,929
Unamortized premium on long-term debt, net of discount		11
Total long-term debt	4,714	3,940	4,714
Line of Credit Facility [Line Items]
Aggregate principal amount		2,000									1,500	500	2,000
Term of credit facility											5 years	364 days
Credit Facility [Abstract]
Reduction in borrowing capacity		500
Additional borrowing available		500
Maximum amount borrowed during the period		0	0
Balance outstanding at the end of the period	0	0	0
Debt Information, by Instrument [Line Items]
Amount under debt tender offer										1,900
Amount debt tendered and accepted for purchase										682
Total purchase price on debt tender										919
Loss on debt tender	229		(229)							229
Face amount of debt				$ 350		$ 500	$ 500	$ 700	$ 300
Interest rate minimum										6.98%
Interest rate maximum										7.875%
Stated interest rate				3.70%	5.05%		1.85%	3.50%	5.05%
Interest payment frequency				Semi-annually	Semi-annually		Semi-annually	Semi-annually	Semi-annually
Debt issuance date				2009-07-01	2009-07-01		2010-11-01	2010-11-01	2010-11-01
Debt maturity date				Aug. 01, 2014	Aug. 01, 2019		Nov. 15, 2015	Mar. 15, 2021	Nov. 15, 2040