Summary of Significant Accounting Policies (Cash Equivalents) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Cash Equivalents Items [Line Items]
Money market funds	$ 948	[1]	$ 615	[1]
Maximum maturity period of cash equivalent investments (in months)	3 months
Midwest Generation LLC
Cash Equivalents Items [Line Items]
Money market funds	$ 103	[2]	$ 75	[2]

[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.