Long-Term Debt (Details 2) (USD $)	1 Months Ended	12 Months Ended					0 Months Ended		1 Months Ended
	Sep. 30, 2011 Credit_Facilities	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2011 Terminated Credit Facility	Aug. 10, 2007 Terminated Credit Facility	Aug. 10, 2007 First Credit Facility	Sep. 30, 2011 First Credit Facility	Sep. 30, 2011 Second Credit Facility	Nov. 30, 2010 $1.919 Billion NGSC debt securities due 2016 to 2036, rates from 6.98% to 7.875%	Sep. 30, 2012 2012 Credit Agreement	Sep. 30, 2012 2011 Credit Agreement
Debt Information, by Instrument [Line Items]
Aggregate principal amount					$ 2,000,000,000	$ 2,000,000,000		$ 1,500,000,000	$ 500,000,000		$ 500,000,000
Debt convenant, debt to capital ratio, maximum (percent)											0.65
Number of credit facilities	2
Term of credit facility							5 years		364 days		364 days	364 days
Reduction in borrowing capacity								500,000,000
Additional borrowing available									500,000,000
Amount under debt tender offer										1,900,000,000
Interest rate minimum										6.98%
Interest rate maximum										7.875%
Amount debt tendered and accepted for purchase										682,000,000
Total purchase price on debt tender										919,000,000
Loss on debt tender		0	0	(229,000,000)						229,000,000
Long-term debt maturities (Table Amounts) [Abstract]
2013		5,000,000
2014		353,000,000
2015		502,000,000
2016		110,000,000
2017		3,000,000
Thereafter		2,951,000,000
Total principal payments		3,924,000,000
Unamortized premium on long-term debt, net of discount		11,000,000
Total long-term debt		$ 3,935,000,000	$ 3,940,000,000