Derivative Instruments (Details 2) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
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)	$ 14	$ 3	$ 5	$ 4
Unrealized Gain (Loss) Recognized				(43)
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)	14	0	5	(2)
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:	0	(3)	16	(7)
Unrealized Gain (Loss) Recognized	33	66	61	39
Additions/Change in value of existing contracts	1	(26)	(36)	(49)
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:	0	(5)	(12)	(5)
Unrealized Gain (Loss) Recognized	(4)		(3)
Additions/Change in value of existing contracts	(39)		(29)
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		0
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	(34)		(54)
Additions/Change in value of existing contracts	0		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	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	0	0	0
Unrealized Gain (Loss) Recognized	0	0	0	0
Additions/Change in value of existing contracts	0	0	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	0	0	0
Unrealized Gain (Loss) Recognized	0		0
Additions/Change in value of existing contracts	18		26
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		0
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	13		13
Additions/Change in value of existing contracts	(59)		(87)
Interest Rate Swap
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	0	0
Interest Rate Swap | 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	0	0	0
Unrealized Gain (Loss) Recognized	0	0	0	0
Additions/Change in value of existing contracts		0	0	0
Interest Rate Swap | 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	0	0	0
Unrealized Gain (Loss) Recognized	0		0
Additions/Change in value of existing contracts	0		0
Interest Rate Swap | 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		0
Interest Rate Swap | 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	0		1
Additions/Change in value of existing contracts	0		0
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
Unrealized Gain (Loss) Recognized	147	35	236	259
Realized Loss to Regulatory Assets:	(62)	(68)	(134)	(146)
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
Unrealized Gain (Loss) Recognized	(147)	(35)	(236)	(259)
Additions/Change in value of existing contracts	62	68	134	146
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)	0	3	0	6
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	0	0	0
Unrealized Gain (Loss) Recognized	0	0	0	0
Additions/Change in value of existing contracts	(2)	0	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	0	0	0
Unrealized Gain (Loss) Recognized	0		0
Additions/Change in value of existing contracts	0		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:		(4)		(8)
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	0		0
Additions/Change in value of existing contracts	0		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
Unrealized Gain (Loss) Recognized	(2)	0	(2)	0
Realized Loss to Regulatory Assets:	0	0	10	9
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
Unrealized Gain (Loss) Recognized	2	0	2	0
Additions/Change in value of existing contracts	0	0	(10)	(9)
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	(3)	16	(7)
Unrealized Gain (Loss) Recognized	33	66	61	39
Additions/Change in value of existing contracts	1	(26)	(36)	(49)
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	(5)	(12)	(5)
Unrealized Gain (Loss) Recognized	(4)		(3)
Additions/Change in value of existing contracts	(21)		(3)
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:		(4)		(8)
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	(21)		(40)
Additions/Change in value of existing contracts	(59)		(87)
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
Unrealized Gain (Loss) Recognized	145	35	234	259
Realized Loss to Regulatory Assets:	(62)	(68)	(124)	(137)
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
Unrealized Gain (Loss) Recognized	(145)	(35)	(234)	(259)
Additions/Change in value of existing contracts	$ 62	$ 68	$ 124	$ 137