Income Taxes (Intercompany Tax-Allocation Agreement) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Tax Benefits Generated, Noncash Distribution of Tax Benefits To Parent	$ 185	$ 222
Midwest Generation LLC [Member] | Hypothetical Tax Return
Operating Loss Carryforwards [Line Items]
Tax Benefits Generated, Noncash Distribution of Tax Benefits To Parent		$ 106