Derivative Instruments and Hedging Activities (Income Statement Impact of Derivative Instruments) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Midwest Generation LLC [Member] | Operating revenues
Derivative [Line Items]
Ineffective portion recognized in earnings	$ (1,000,000)		$ 0		$ 4,000,000
Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Ineffective portion recognized in earnings	(1,000,000)		0		4,000,000
Cash Flow Hedges | Midwest Generation LLC [Member]
Derivative [Line Items]
Beginning of period derivative gains (losses)	(2,000,000)	[1]	34,000,000
Effective portion of changes in fair value	(2,000,000)		7,000,000
End of period derivative gains (losses)	0	[1]	(2,000,000)	[1]
Cash Flow Hedges | Midwest Generation LLC [Member] | Operating revenues
Derivative [Line Items]
Reclassification to earnings	4,000,000		(43,000,000)
Economic Hedges | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	(9,000,000)		31,000,000
Economic Hedges | Fuel
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	0		2,000,000
Economic Hedges | Interest rate contracts
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	6,000,000		0
Economic Hedges | Midwest Generation LLC [Member] | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	(9,000,000)		31,000,000
Economic Hedges | Midwest Generation LLC [Member] | Fuel
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	0		2,000,000
Trading Activities | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	75,000,000		68,000,000
Commodity contracts | Cash Flow Hedges
Derivative [Line Items]
Beginning of period derivative gains (losses)	(1,000,000)	[2]	35,000,000
Effective portion of changes in fair value	(2,000,000)		5,000,000
End of period derivative gains (losses)	0	[2]	(1,000,000)	[2]
Commodity contracts | Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Reclassification to earnings	3,000,000		(41,000,000)
Commodity contracts | Cash Flow Hedges | Interest Expense [Member]
Derivative [Line Items]
Reclassification to earnings	0		0
Interest rate contracts | Cash Flow Hedges
Derivative [Line Items]
Beginning of period derivative gains (losses)	(118,000,000)	[2]	(90,000,000)
Effective portion of changes in fair value	55,000,000		(28,000,000)
End of period derivative gains (losses)	(58,000,000)	[2]	(118,000,000)	[2]
Interest rate contracts | Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Reclassification to earnings	0		0
Interest rate contracts | Cash Flow Hedges | Interest Expense [Member]
Derivative [Line Items]
Reclassification to earnings	$ 5,000,000		$ 0

[1] Unrealized derivative gains (losses) are before income taxes. Amounts recorded in AOCI include commodity contracts. For additional information, see Note 11—Accumulated Other Comprehensive Loss.

[2] 1 Unrealized derivative gains (losses) are before income taxes. Amounts recorded in AOCI include commodity and interest rate contracts. For additional information, see Note 11—Accumulated Other Comprehensive Loss.