Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 months segment	Dec. 31, 2011
Summary of Significant Accounting Policies [Line Items]
Number of segments for financial reporting purposes	2
Cash Equivalents
Money market funds	$ 1,200	$ 1,300
Maximum maturity period of cash equivalent investments (in months)	3
Cash reclassified to accounts payable under cash management program	178	220
Inventory
Coal, gas, fuel oil and other raw materials	170	211
Spare parts, materials and supplies	409	413
Total inventory	579	624
Midwest Generation LLC. | Letters of Credit
Restricted Cash and Deposits
Restricted deposits	74
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 deposits	$ 97	$ 97