Derivative Instruments and Hedging Activities (Income Statement Impact of Derivative Instruments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Midwest Generation LLC | Operating revenues
Derivative [Line Items]
Ineffective portion recognized in earnings	$ 0		$ 4		$ 7
Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Ineffective portion recognized in earnings	0		4		6
Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Beginning of period derivative gains (losses)	34	[1]	37	[1]
Effective portion of changes in fair value	7	[1]	38	[1]
End of period derivative gains (losses)	(2)	[1]	34	[1]
Cash Flow Hedges | Midwest Generation LLC | Operating revenues
Derivative [Line Items]
Reclassification to earnings	(43)	[1]	(41)	[1]
Economic Hedges | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	31		5
Economic Hedges | Fuel
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	2		3
Economic Hedges | Midwest Generation LLC | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	31		2
Economic Hedges | Midwest Generation LLC | Fuel
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	2		3
Trading Activities | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	68		76
Commodity contracts | Cash Flow Hedges
Derivative [Line Items]
Beginning of period derivative gains (losses)	35	[2]	43	[2]
Effective portion of changes in fair value	5	[2]	55	[2]
End of period derivative gains (losses)	(1)	[2]	35	[2]
Commodity contracts | Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Reclassification to earnings	(41)	[2]	(63)	[2]
Interest rate contracts | Cash Flow Hedges
Derivative [Line Items]
Beginning of period derivative gains (losses)	(90)	[2]	(16)	[2]
Effective portion of changes in fair value	(28)	[2]	(74)	[2]
End of period derivative gains (losses)	(118)	[2]	(90)	[2]
Interest rate contracts | Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Reclassification to earnings	$ 0	[2]	$ 0	[2]

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

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