Accumulated Other Comprehensive Income (Reclassifications Out of AOCI) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenues	$ 315		$ 324	$ 622		$ 667
Interest Expense	(22)		(85)	(39)		(170)
Prior service costs	(109)		(147)	(207)		(285)
Benefit for income taxes	25		81	28		157
Net Income (loss)	(84)		(104)	(166)		(186)
Reclassification out of AOCI | Unrealized gains and losses on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Benefit for income taxes	0			(1)
Net Income (loss)	0			2
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	(2)			1
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	2			2
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	(2)			(2)
Reclassification out of AOCI | Amortization of retirement benefit items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Benefit for income taxes	0			1
Net Income (loss)	(1)			(2)
Reclassification out of AOCI | Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Plant operations and Administrative and general	1	[1]		(1)	[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			2
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	0			1
Midwest Generation LLC
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenues	203		213	382		446
Interest Expense	(7)		(8)	(13)		(17)
Prior service costs	(74)		(120)	(141)		(229)
Benefit for income taxes	(1)		27	(1)		32
Net Income (loss)	(74)		(42)	(148)		(51)
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]
Benefit for income taxes	1			0
Net Income (loss)	(1)			1
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	(2)			1
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	(1)			(1)
Midwest Generation LLC | Reclassification out of AOCI | Amortization of retirement benefit items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Benefit for income taxes	1			1
Net Income (loss)	0			(1)
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	$ 0			$ (1)

[1] For the three and six months ended June 30, 2013, $1 million and $2 million were reclassified from AOCI to plant operations, respectively, and none and $1 million from AOCI to administrative and general expenses