Derivative Instruments and Hedging Activities (Balance Sheet Disclosures) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	$ 35		$ 53
Derivative Assets, Long Term	21		37
Derivative Assets	56		90
Derivative Liabilities, Long Term	69		118
Derivative Liabilities	69		118
Margin and collateral deposits	85		61
Midwest Generation LLC
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	1		2
Derivative Liabilities, Short-term	2		3
Electricity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets	54		90
Electricity contracts | Midwest Generation LLC
Derivative, Fair Value, Net [Abstract]
Derivative Assets	1		2
Natural gas contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets	0		0
Coal contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets	0		0
Gross Derivative Contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	127		155
Derivative Assets, Long Term	42		53
Derivative Liabilities, Short-term	87		109
Derivative Liabilities, Long Term	91		134
Gross Derivative Contract | Electricity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	96		120
Derivative Assets, Long Term	36		52
Gross Derivative Contract | Electricity contracts | Midwest Generation LLC
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	5		12
Derivative Liabilities, Short-term	6		13
Gross Derivative Contract | Natural gas contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	30		33
Derivative Assets, Long Term	4		1
Gross Derivative Contract | Coal contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	1		2
Derivative Assets, Long Term	0		0
Netting and Collateral Received
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	(92)		(102)
Derivative Assets, Long Term	(21)		(16)
Derivative Liabilities, Short-term	(87)		(109)
Derivative Liabilities, Long Term	(22)		(16)
Netting and Collateral Received | Electricity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	(61)		(67)
Derivative Assets, Long Term	(17)		(15)
Netting and Collateral Received | Electricity contracts | Midwest Generation LLC
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	(4)		(10)
Derivative Liabilities, Short-term	(4)		(10)
Netting and Collateral Received | Natural gas contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	(30)		(33)
Derivative Assets, Long Term	(4)		(1)
Netting and Collateral Received | Coal contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	(1)		(2)
Derivative Assets, Long Term	0		0
Net Derivative Contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	35		53
Derivative Assets, Long Term	21		37
Derivative Liabilities, Short-term	0		0
Derivative Liabilities, Long Term	69		118
Net Derivative Contract | Electricity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	35		53
Derivative Assets, Long Term	19		37
Net Derivative Contract | Electricity contracts | Midwest Generation LLC
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	1		2
Derivative Liabilities, Short-term	2		3
Net Derivative Contract | Natural gas contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	0		0
Derivative Assets, Long Term	0		0
Net Derivative Contract | Coal contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	0		0
Derivative Assets, Long Term	0		0
Subject to Master Netting Agreement
Derivative, Fair Value, Net [Abstract]
Derivative Assets	54
Derivative Liabilities	0		0
Subject to Master Netting Agreement | Electricity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities	0		0
Subject to Master Netting Agreement | Natural gas contracts
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities	0		0
Subject to Master Netting Agreement | Coal contract
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities	0		0
Subject to Master Netting Agreement | Gross Derivative Contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	127
Derivative Assets, Long Term	40
Derivative Liabilities, Short-term	87		109
Derivative Liabilities, Long Term	22		16
Subject to Master Netting Agreement | Gross Derivative Contract | Electricity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	60		71
Derivative Liabilities, Long Term	17		15
Subject to Master Netting Agreement | Gross Derivative Contract | Natural gas contracts
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	26		36
Derivative Liabilities, Long Term	5		1
Subject to Master Netting Agreement | Gross Derivative Contract | Coal contract
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	1		2
Derivative Liabilities, Long Term	0		0
Subject to Master Netting Agreement | Netting and Collateral Received
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	(92)
Derivative Assets, Long Term	(21)
Derivative Liabilities, Short-term	(87)		(109)
Derivative Liabilities, Long Term	(22)		(16)
Subject to Master Netting Agreement | Netting and Collateral Received | Electricity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	(60)		(71)
Derivative Liabilities, Long Term	(17)		(15)
Subject to Master Netting Agreement | Netting and Collateral Received | Natural gas contracts
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	(26)		(36)
Derivative Liabilities, Long Term	(5)		(1)
Subject to Master Netting Agreement | Netting and Collateral Received | Coal contract
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	(1)		(2)
Derivative Liabilities, Long Term	0		0
Subject to Master Netting Agreement | Net Derivative Contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	35
Derivative Assets, Long Term	19
Derivative Liabilities, Short-term	0		0
Derivative Liabilities, Long Term	0		0
Subject to Master Netting Agreement | Net Derivative Contract | Electricity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	0		0
Derivative Liabilities, Long Term	0		0
Subject to Master Netting Agreement | Net Derivative Contract | Natural gas contracts
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	0		0
Derivative Liabilities, Long Term	0		0
Subject to Master Netting Agreement | Net Derivative Contract | Coal contract
Derivative, Fair Value, Net [Abstract]
Derivative Liabilities, Short-term	0		0
Derivative Liabilities, Long Term	0		0
Not Subject to Master Netting Agreement
Derivative, Fair Value, Net [Abstract]
Derivative Assets	2	[1]
Derivative Liabilities	69	[1]	118	[1]
Not Subject to Master Netting Agreement | Gross Derivative Contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	0	[1]
Derivative Assets, Long Term	2	[1]
Derivative Liabilities, Short-term	0	[1]	0	[1]
Derivative Liabilities, Long Term	69	[1]	118	[1]
Not Subject to Master Netting Agreement | Netting and Collateral Received
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	0	[1]
Derivative Assets, Long Term	0	[1]
Derivative Liabilities, Short-term	0	[1]	0	[1]
Derivative Liabilities, Long Term	0	[1]	0	[1]
Not Subject to Master Netting Agreement | Net Derivative Contract
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	0	[1]
Derivative Assets, Long Term	2	[1]
Derivative Liabilities, Short-term	0	[1]	0	[1]
Derivative Liabilities, Long Term	69	[1]	118	[1]
Net accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative collateral, net accounts receivable	$ 42		$ 44

[1] EME's interest rate swaps are not subject to master netting agreements and do not require EME to post collateral.