Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Assets at Fair Value
Money market funds	$ 948	[1]			$ 948	[1]			$ 615	[1]
Derivative contracts	56				56				90
Total assets	1,004				1,004				705
Liabilities at Fair Value
Total liabilities	69				69				118
Midwest Generation LLC
Assets at Fair Value
Money market funds	103	[2]			103	[2]			75	[2]
Total assets	104				104				77
Liabilities at Fair Value
Total liabilities	2				2				3
Electricity
Assets at Fair Value
Derivative contracts	54				54				90
Liabilities at Fair Value
Derivative contracts	0				0				0
Electricity | Midwest Generation LLC
Assets at Fair Value
Derivative contracts	1				1				2
Liabilities at Fair Value
Derivative contracts	2				2				3
Natural gas
Assets at Fair Value
Derivative contracts	0				0				0
Liabilities at Fair Value
Derivative contracts									0
Interest rate
Assets at Fair Value
Interest rate derivative contracts	2				2
Liabilities at Fair Value
Derivative contracts	69				69				118
Level 1
Assets at Fair Value
Money market funds	948	[1]			948	[1]			615	[1]
Total assets	951				951				615
Liabilities at Fair Value
Total liabilities	0				0				3
Level 1 | Midwest Generation LLC
Assets at Fair Value
Money market funds	103	[2]			103	[2]			75	[2]
Total assets	103				103				75
Liabilities at Fair Value
Total liabilities	0				0				0
Level 1 | Electricity
Assets at Fair Value
Derivative contracts	0				0				0
Liabilities at Fair Value
Derivative contracts	0				0				0
Level 1 | Electricity | Midwest Generation LLC
Assets at Fair Value
Derivative contracts	0				0				0
Liabilities at Fair Value
Derivative contracts	0				0				0
Level 1 | Natural gas
Assets at Fair Value
Derivative contracts	3				3
Liabilities at Fair Value
Derivative contracts									3
Level 1 | Interest rate
Assets at Fair Value
Interest rate derivative contracts	0				0
Liabilities at Fair Value
Derivative contracts	0				0				0
Level 2
Assets at Fair Value
Money market funds	0	[1]			0	[1]			0	[1]
Total assets	37				37				41
Liabilities at Fair Value
Total liabilities	70				70				124
Level 2 | Midwest Generation LLC
Assets at Fair Value
Money market funds	0	[2]			0	[2]			0	[2]
Total assets	1				1				2
Liabilities at Fair Value
Total liabilities	2				2				3
Level 2 | Electricity
Assets at Fair Value
Derivative contracts	35				35				41
Liabilities at Fair Value
Derivative contracts	1				1				6
Level 2 | Electricity | Midwest Generation LLC
Assets at Fair Value
Derivative contracts	1				1				2
Liabilities at Fair Value
Derivative contracts	2				2				3
Level 2 | Natural gas
Assets at Fair Value
Derivative contracts	0				0
Liabilities at Fair Value
Derivative contracts									0
Level 2 | Interest rate
Assets at Fair Value
Interest rate derivative contracts	2				2
Liabilities at Fair Value
Derivative contracts	69				69				118
Level 3
Assets at Fair Value
Money market funds	0	[1]			0	[1]			0	[1]
Total assets	26				26				52
Liabilities at Fair Value
Total liabilities	5				5				1
Fair Value Disclosures Level 3 [Roll Forward]
Fair value of net assets at beginning of period	26		41		51		83
Total realized/unrealized gains (losses)
Included in earnings	5	[3]	12	[3]	(14)	[3]	20	[3]
Included in AOCI	0	[4]	(1)	[4]	0	[4]	1	[4]
Purchases	8		8		27		27
Settlements	(18)		(29)		(43)		(49)
Transfers out of Level 3	0		0		0		(51)
Fair value of net assets at end of period	21		31		21		31
Change during the period in unrealized gains (losses) related to assets and liabilities held at end of period	(1)		(7)		(18)		0
Level 3 | Electricity
Assets at Fair Value
Derivative contracts	26				26				52
Liabilities at Fair Value
Derivative contracts	5				5				1
Level 3 | Natural gas
Assets at Fair Value
Derivative contracts	0				0
Liabilities at Fair Value
Derivative contracts									0
Level 3 | Interest rate
Assets at Fair Value
Interest rate derivative contracts	0				0
Liabilities at Fair Value
Derivative contracts	0				0				0
Netting and Collateral
Assets at Fair Value
Money market funds	0	[1],[5]			0	[1],[5]			0	[1],[5]
Total assets	(10)	[5]			(10)	[5]			(3)	[5]
Liabilities at Fair Value
Total liabilities	(6)	[5]			(6)	[5]			(10)	[5]
Netting and Collateral | Midwest Generation LLC
Assets at Fair Value
Money market funds	0	[2],[6]			0	[2],[6]			0	[2],[6]
Total assets	0	[6]			0	[6]			0	[6]
Liabilities at Fair Value
Total liabilities	0	[6]			0	[6]			0	[6]
Netting and Collateral | Electricity
Assets at Fair Value
Derivative contracts	(7)	[5]			(7)	[5]			(3)	[5]
Liabilities at Fair Value
Derivative contracts	(6)	[5]			(6)	[5]			(7)	[5]
Netting and Collateral | Electricity | Midwest Generation LLC
Assets at Fair Value
Derivative contracts	0	[6]			0	[6]			0	[6]
Liabilities at Fair Value
Derivative contracts	0	[6]			0	[6]			0	[6]
Netting and Collateral | Natural gas
Assets at Fair Value
Derivative contracts	(3)	[5]			(3)	[5]
Liabilities at Fair Value
Derivative contracts									(3)	[5]
Netting and Collateral | Interest rate
Assets at Fair Value
Interest rate derivative contracts	0	[5]			0	[5]
Liabilities at Fair Value
Derivative contracts	$ 0	[5]			$ 0	[5]			$ 0	[5]

[1] Money market funds are included in cash and cash equivalents on EME's consolidated balance sheets.

[2] Money market funds are included in cash and cash equivalents on Midwest Generation's consolidated balance sheets.

[3] Reported in operating revenues on EME's consolidated statements of operations.

[4] Included in reclassification adjustments in EME's consolidated statement of OCI.

[5] Represents cash collateral and the impact of netting across the levels of the fair value hierarchy. Netting among positions classified within the same level is included in that level.

[6] Represents the impact of netting across the levels of the fair value hierarchy. Netting among positions classified within the same level is included in that level.