Derivative Instruments (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)	$ 1	$ 14
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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)	0	0
Interest rate swaps
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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)	0	0
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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)	0	0
Designated as Hedging Instrument [Member]
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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)	1	14	(4)	5
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				16
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				(12)
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)			(4)	5
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				16
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				(12)
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)			0	0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | Interest rate swaps
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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)			0	0
Designated as Hedging Instrument [Member] | Interest rate swaps | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | Interest rate swaps | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | Interest rate swaps | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Gain (Loss) Recognized in AOCL (Effective Portion)			0	0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Designated as Hedging Instrument [Member] | 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
Hedging Relationship; Effective Gain (Loss) Reclassified to:				0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		33		61
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(104)	1	(221)	(36)
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		(4)		(3)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	100	(26)	217	(14)
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(1)		(1)
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	24	(23)	82	(41)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(18)	(40)	(41)	(54)
Not Designated as Hedging Instrument [Member] | Interest 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	(20)	0	(20)	0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		33		61
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(104)	1	(221)	(36)
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		(4)		(3)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	95	(39)	206	(29)
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0		0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	7	(34)	62	(54)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | Power Contracts | Interest 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	0	0	0	0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		0		0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		0		0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	5	13	11	16
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0		0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	12	11	17	12
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(18)	(40)	(41)	(54)
Not Designated as Hedging Instrument [Member] | FTRs | Interest 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	0	0	0	0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | Interest rate swaps | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		0		0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | Interest rate swaps | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		0		0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	(1)
Not Designated as Hedging Instrument [Member] | Interest rate swaps | 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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0		0
Not Designated as Hedging Instrument [Member] | Interest rate swaps | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	0	0	0	1
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | Interest rate swaps | Interest 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	(20)	0	(20)	0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		0		0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:		0		0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(1)		(1)
Not Designated as Hedging Instrument [Member] | 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	5	0	3	0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
Not Designated as Hedging Instrument [Member] | Other | Interest 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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	0	0	0	0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	$ 0	$ 0	$ 0	$ 0