Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2012 segment	Dec. 31, 2011
Summary of Significant Accounting Policies [Line Items]
Number of segments for financial reporting purposes	2
Cash Equivalents
Money market funds	$ 982	$ 1,300
Maximum maturity period of cash equivalent investments (in months)	3 months
Cash reclassified to accounts payable under cash management program	132	220
Restricted Cash and Deposits
Restricted deposits	97	51
Inventory
Coal, gas, fuel oil and other raw materials	160	211
Spare parts, materials and supplies	407	413
Total inventory	567	624
Midwest Generation LLC. | Letters of Credit
Restricted Cash and Deposits
Restricted deposits	51
Electric Utility
Summary of Significant Accounting Policies [Line Items]
Supply of electricity, area covered (square mile)	50,000
Wind Financing
Restricted Cash and Deposits
Restricted cash and cash equivalents	$ 97	$ 97