Income Taxes (Intercompany Tax-Allocation Agreement) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2012 Midwest Generation LLC		Dec. 31, 2012 Midwest Generation LLC Hypothetical Tax Return	Dec. 31, 2011 Due to Affiliates Midwest Generation LLC
Operating Loss Carryforwards [Line Items]
Tax allocation payments, paid for reallocation of tax obligations	$ 185
Non-cash distribution to parent	222	[1]	106	[2]	106
Income tax allocation agreements						$ 13

[1] During the fourth quarter of 2012, EME recorded a non-cash distribution to EIX related to the tax-allocation agreement. See Note 7—Income Taxes—EME—Current and Deferred Taxes.

[2] During 2012, Midwest Generation recorded a non-cash distribution to its parent related to the tax-allocation agreement. See Note 7—Income Taxes—Midwest Generation—Current and Deferred Taxes.