Fair Value Disclosures - (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Additional Fair Value Elements [Line Items]
Carrying amount of long term debt including current maturities	$ 13,390,000,000		$ 12,941,000,000
Schedule Of Available For Sale Securities [Line Items]
Fair Value	1,765,000,000		1,630,000,000
Unrealized Losses	19,000,000		31,000,000
Unrealized Gains	551,000,000		444,000,000
Aggregate fair value of investments related to unrealized losses
Aggregate fair value of investments related to unrealized losses	155,000,000		136,000,000
Fair value of available for sale debt securities by contractual maturity
Due in one year or less	34,000,000
Due after one through five years	217,000,000
Due after five through 10 years	175,000,000
Due after 10 years	94,000,000
Total	520,000,000
Sales of available for sale securities
Proceeds	304,000,000	1,744,000,000
Realized gains	7,000,000	9,000,000
Realized losses	3,000,000	4,000,000
Nuclear decommissioning trust funds
Common stock equity	1,160,000,000		1,033,000,000
Preferred stock and other equity	43,000,000		29,000,000
Corporate debt	90,000,000		86,000,000
U.S. state and municipal debt	124,000,000		128,000,000
U.S. and foreign government debt	293,000,000		284,000,000
Money market funds and other	52,000,000		87,000,000
Total nuclear decommissioning trust funds	1,762,000,000		1,647,000,000
Derivatives
Commodity forward contracts	8,000,000		5,000,000
Other marketable securities
Money market and other	18,000,000		20,000,000
Total assets	1,788,000,000		1,672,000,000
Derivatives
Commodity forward contracts	796,000,000		692,000,000
Interest rate contracts	49,000,000		93,000,000
Contingent value obligations derivatives	3,000,000		14,000,000
Total liabilities	848,000,000		799,000,000
Common Stock Outstanding - Amount
Schedule Of Available For Sale Securities [Line Items]
Fair Value	1,160,000,000		1,033,000,000
Unrealized Losses	16,000,000		29,000,000
Unrealized Gains	508,000,000		401,000,000
Preferred stock and other equity
Schedule Of Available For Sale Securities [Line Items]
Fair Value	43,000,000		29,000,000
Unrealized Gains	14,000,000		11,000,000
Corporate debt
Schedule Of Available For Sale Securities [Line Items]
Fair Value	90,000,000		86,000,000
Unrealized Gains	7,000,000		6,000,000
U.S. state and municipal debt
Schedule Of Available For Sale Securities [Line Items]
Fair Value	124,000,000		128,000,000
Unrealized Losses	2,000,000		2,000,000
Unrealized Gains	7,000,000		7,000,000
U.S. and foreign government debt
Schedule Of Available For Sale Securities [Line Items]
Fair Value	293,000,000		284,000,000
Unrealized Gains	14,000,000		18,000,000
Money market funds and other securities
Schedule Of Available For Sale Securities [Line Items]
Fair Value	55,000,000		70,000,000
Unrealized Losses	1,000,000
Unrealized Gains	1,000,000		1,000,000
Level 1
Nuclear decommissioning trust funds
Common stock equity	1,160,000,000		1,033,000,000
Preferred stock and other equity	33,000,000		28,000,000
U.S. and foreign government debt	129,000,000		87,000,000
Money market funds and other	1,000,000
Total nuclear decommissioning trust funds	1,323,000,000		1,148,000,000
Other marketable securities
Money market and other	18,000,000		20,000,000
Total assets	1,341,000,000		1,168,000,000
Level 2
Additional Fair Value Elements [Line Items]
Long-term Debt, Fair Value	15,400,000,000		15,300,000,000
Nuclear decommissioning trust funds
Preferred stock and other equity	10,000,000		1,000,000
Corporate debt	90,000,000		86,000,000
U.S. state and municipal debt	124,000,000		128,000,000
U.S. and foreign government debt	164,000,000		197,000,000
Money market funds and other	51,000,000		87,000,000
Total nuclear decommissioning trust funds	439,000,000		499,000,000
Derivatives
Commodity forward contracts	8,000,000		5,000,000
Other marketable securities
Total assets	447,000,000		504,000,000
Derivatives
Commodity forward contracts	769,000,000		668,000,000
Interest rate contracts	49,000,000		93,000,000
Contingent value obligations derivatives	3,000,000		14,000,000
Total liabilities	821,000,000		775,000,000
Level 3
Derivatives
Commodity forward contracts	27,000,000		24,000,000
Total liabilities	27,000,000		24,000,000
Fair value hierarchy
Derivatives, net at beginning of period	24,000,000	36,000,000
Deferred as regulatory assets and liabilities, net	3,000,000	(4,000,000)
Derivatives, net at end of period	$ 27,000,000	$ 32,000,000
Level 3 | Derivative financial instruments, liabilities | Maximum
Fair Value Inputs Liabilities Quantitative Information [Line Items]
Fair value inputs, price per MMBtu	4.528
Level 3 | Derivative financial instruments, liabilities | Minimum
Fair Value Inputs Liabilities Quantitative Information [Line Items]
Fair value inputs, price per MMBtu	4.111