Derivative Instruments (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
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)	$ (5)	$ (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
Hedging Relationship Effective Gain (Loss) Reclassified to:		16
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	0	29
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(117)	(37)
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)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	118	12
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
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:	58	(18)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(24)	(15)
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)	(5)	(9)
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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	0	29
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(117)	(37)
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)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	112	10
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
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:	55	(20)
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	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
Hedging Relationship; 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
Hedging Relationship Effective Gain (Loss) Reclassified to:		0
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	0	0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	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
Hedging Relationship Effective Gain (Loss) Reclassified to:		0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	6	3
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
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	1
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	(24)	(15)
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
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
Not in a Hedging Relationship; Unrealized Gain (Loss) Recognized in:	0	0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	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
Hedging Relationship Effective Gain (Loss) Reclassified to:		0
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	0	(1)
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
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:	(2)	1
Not in a Hedging Relationship; Realized Gain (Loss) Reclassified to:	$ 0	$ 0