Accumulated Other Comprehensive Income (Reclassifications Out of AOCI) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenues	$ 385		$ 340	$ 1,007		$ 1,007
Prior service costs	(89)		(105)	(296)		(389)
Provision (benefit) for income taxes	(3)		47	25		204
Net Income (loss)	(448)		(162)	(614)		(348)
Reclassification out of AOCI | Unrealized gains and losses on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision (benefit) for income taxes	4			3
Net Income (loss)	(7)			(5)
Reclassification out of AOCI | Unrealized gains and losses on cash flow hedges | Electricity contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenues	(5)			(4)
Reclassification out of AOCI | Unrealized gains and losses on cash flow hedges | Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest Expense	(6)			(4)
Reclassification out of AOCI | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service costs	0			(2)
Reclassification out of AOCI | Amortization of retirement benefit items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision (benefit) for income taxes	1			2
Net Income (loss)	(1)			(3)
Reclassification out of AOCI | Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Plant operations and Administrative and general	(2)	[1]		(3)	[1]
Reclassification out of AOCI | Actuarial losses | Plant operations
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Plant operations and Administrative and general	1			3
Reclassification out of AOCI | Actuarial losses | Administrative and general expenses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Plant operations and Administrative and general	1			2
Midwest Generation LLC
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenues	232		253	614		699
Interest Expense	(4)		(8)	(17)		(25)
Prior service costs	(49)		(73)	(190)		(302)
Provision (benefit) for income taxes	1		7	0		39
Net Income (loss)	(471)		(12)	(619)		(63)
Midwest Generation LLC | Reclassification out of AOCI | Unrealized gains and losses on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision (benefit) for income taxes	2			2
Net Income (loss)	(3)			(2)
Midwest Generation LLC | Reclassification out of AOCI | Unrealized gains and losses on cash flow hedges | Electricity contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenues	(5)			(4)
Midwest Generation LLC | Reclassification out of AOCI | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service costs	0			(1)
Midwest Generation LLC | Reclassification out of AOCI | Amortization of retirement benefit items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision (benefit) for income taxes	0			1
Net Income (loss)	(1)			(2)
Midwest Generation LLC | Reclassification out of AOCI | Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Plant operations and Administrative and general	$ (1)			$ (2)

[1] For the three and nine months ended September 30, 2013, $1 million and $3 million were reclassified from AOCI to plant operations, respectively, and $1 million and $2 million from AOCI to administrative and general expenses