Derivative Instruments (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
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 AOCI (Effective Portion)	$ (2)	$ 0	$ (6)	$ 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] | 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 AOCI (Effective Portion)	(2)	0	(6)	4
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] | 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 AOCI (Effective Portion)	0	0	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] | 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 AOCI (Effective Portion)	0	0	0	1
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] | 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 AOCI (Effective Portion)	0	0	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
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:		27		88
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(27)	(5)	(248)	(41)
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:		3		(1)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	52	(20)	278	(35)
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:	2	(19)	84	(61)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(10)	(22)	(51)	(77)
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:	3		2
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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	6		6
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:		27		88
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(27)	(5)	(248)	(41)
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:		3		(1)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	46	(39)	260	(69)
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	(11)	69	(65)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
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 | 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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	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:	6	20	18	36
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:	(5)	(9)	12	2
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(10)	(22)	(51)	(77)
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 | 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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	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	(1)	0	(2)
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	1	0	2
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	0	0	0
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 | 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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	6		6
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 | 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	3	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:	3		2
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
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	$ 0		$ 0