Derivative Instruments and Hedging Activities (Income Statement Impact of Derivative Instruments) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Midwest Generation LLC | Operating revenues
Derivative [Line Items]
Ineffective portion recognized in earnings	$ (1,000,000)	$ 1,000,000	$ (1,000,000)	$ 2,000,000
Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Ineffective portion recognized in earnings	(1,000,000)	0	(1,000,000)	1,000,000
Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Beginning of period derivative gains (losses)			(2,000,000)	34,000,000
Effective portion of changes in fair value			(3,000,000)	20,000,000
End of period derivative gains (losses)	(4,000,000)	23,000,000	(4,000,000)	23,000,000
Cash Flow Hedges | Midwest Generation LLC | Operating revenues
Derivative [Line Items]
Reclassification to operating revenues			1,000,000	(31,000,000)
Economic Hedges | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	(2,000,000)	6,000,000	(2,000,000)	17,000,000
Economic Hedges | Fuel
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	0	(6,000,000)	0	(1,000,000)
Economic Hedges | Midwest Generation LLC | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	(2,000,000)	5,000,000	(2,000,000)	16,000,000
Economic Hedges | Midwest Generation LLC | Fuel
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	0	(6,000,000)	0	(1,000,000)
Trading Activities | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	13,000,000	30,000,000	42,000,000	50,000,000
Commodity contracts | Cash Flow Hedges
Derivative [Line Items]
Beginning of period derivative gains (losses)			(1,000,000)	35,000,000
Effective portion of changes in fair value			(3,000,000)	19,000,000
End of period derivative gains (losses)	(3,000,000)	21,000,000	(3,000,000)	21,000,000
Commodity contracts | Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Reclassification to operating revenues			1,000,000	(33,000,000)
Commodity contracts | Cash Flow Hedges | Interest Expense [Member] [Member]
Derivative [Line Items]
Reclassification to operating revenues			0	0
Interest rate contracts | Cash Flow Hedges
Derivative [Line Items]
Beginning of period derivative gains (losses)			(118,000,000)	(90,000,000)
Effective portion of changes in fair value			44,000,000	(24,000,000)
End of period derivative gains (losses)	(72,000,000)	(114,000,000)	(72,000,000)	(114,000,000)
Interest rate contracts | Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Reclassification to operating revenues			0	0
Interest rate contracts | Cash Flow Hedges | Interest Expense [Member] [Member]
Derivative [Line Items]
Reclassification to operating revenues			$ 2,000,000	$ 0