Derivative Instruments (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Gain (Loss) Recognized in AOCL (Effective Portion)	$ 12		$ 23
Unrealized Gain (Loss) Recognized	0		(129)		0
Purchase Power Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:	16	[1]	(7)	[1]
Unrealized Gain (Loss) Recognized	120		86
Additions/Change in value of existing contracts	(159)		(104)
Revenue
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:	(12)	[1]	(4)	[1]
Unrealized Gain (Loss) Recognized	(3)
Additions/Change in value of existing contracts	84
Fuel Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:			(14)	[1]
Other Operating Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Unrealized Gain (Loss) Recognized	(64)
Additions/Change in value of existing contracts	(157)
Regulatory Asset
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Additions/Change in value of existing contracts	(241)	[2]	(271)	[2]
Realized Gain to Regulatory Assets:	207	[2]	181	[2]
Derivative Instrument
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Additions/Change in value of existing contracts	241	[2]	271	[2]
Realized Loss to Regulatory Assets:	(207)	[2]	(181)	[2]
Power Contracts
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Gain (Loss) Recognized in AOCL (Effective Portion)	11		12
Power Contracts | Purchase Power Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:	16	[1]	(7)	[1]
Unrealized Gain (Loss) Recognized	120		86
Additions/Change in value of existing contracts	(159)		(104)
Power Contracts | Revenue
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:	(12)	[1]	(4)	[1]
Unrealized Gain (Loss) Recognized	(3)
Additions/Change in value of existing contracts	17
Power Contracts | Fuel Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:			0	[1]
Power Contracts | Other Operating Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Unrealized Gain (Loss) Recognized	(52)
Additions/Change in value of existing contracts	0
FTRs
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Gain (Loss) Recognized in AOCL (Effective Portion)	0		0
FTRs | Purchase Power Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:	0	[1]	0	[1]
Unrealized Gain (Loss) Recognized	0		0
Additions/Change in value of existing contracts	0		0
FTRs | Revenue
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:	0	[1]	0	[1]
Unrealized Gain (Loss) Recognized	0
Additions/Change in value of existing contracts	67
FTRs | Fuel Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:			0	[1]
FTRs | Other Operating Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Unrealized Gain (Loss) Recognized	(14)
Additions/Change in value of existing contracts	(157)
NUGs | Regulatory Asset
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Additions/Change in value of existing contracts	(254)	[2]	(280)	[2]
Realized Gain to Regulatory Assets:	202	[2]	181	[2]
NUGs | Derivative Instrument
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Additions/Change in value of existing contracts	254	[2]	280	[2]
Realized Loss to Regulatory Assets:	(202)	[2]	(181)	[2]
Other
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Gain (Loss) Recognized in AOCL (Effective Portion)	1		11
Other | Purchase Power Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:	0	[1]	0	[1]
Unrealized Gain (Loss) Recognized	0		0
Additions/Change in value of existing contracts	0		0
Other | Revenue
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:	0	[1]	0	[1]
Unrealized Gain (Loss) Recognized	0
Additions/Change in value of existing contracts	0
Other | Fuel Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Effective Gain (Loss) Reclassified to:			(14)	[1]
Other | Other Operating Expense
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Unrealized Gain (Loss) Recognized	2
Additions/Change in value of existing contracts	0
Other | Regulatory Asset
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Additions/Change in value of existing contracts	13	[2]	9	[2]
Realized Gain to Regulatory Assets:	5	[2]	0	[2]
Other | Derivative Instrument
Effect of derivative instruments on the statements of income and comprehensive income for instruments designated in cash flow hedging relationships and not in hedging relationships
Additions/Change in value of existing contracts	(13)	[2]	(9)	[2]
Realized Loss to Regulatory Assets:	$ (5)	[2]	$ 0	[2]

[1] The ineffective portion was immaterial.

[2] Changes in the fair value of certain contracts are deferred for future recovery from (or refund to) customers.