Summary of Significant Accounting Policies (Cash Equivalents) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	$ 615	[1]	$ 1,179	[1]
Maximum maturity period of cash equivalent investments (in months)	3 months
Restricted cash and cash equivalents			97
Midwest Generation LLC
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	75	[2]	195	[2]
Midwest Generation LLC | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted deposits	4		3
Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted deposits	$ 49

[1] Money market funds are included in cash and cash equivalents and in restricted 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.