Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011	Dec. 31, 2011		Dec. 31, 2010
Summary of Significant Accounting Policies [Line Items]
Corporate cash and cash equivalents, without contractual dividend restrictions	$ 879
Cash and cash equivalents	1,009		833	1,300		1,075
Deferred tax benefits	323
Revision to correct error in presentation	166		219
Cash Equivalents
Money market funds	873	[1]		1,207	[1]
Maximum maturity period of cash equivalent investments (in months)	3 months
Restricted cash and cash equivalents	97			97
Inventory
Coal, fuel oil and other raw materials	142			188
Spare parts, materials and supplies	90			86
Total inventory	232			274
Vendor Financed Property, Plant and Equipment
Summary of Significant Accounting Policies [Line Items]
Revision to correct error in presentation	19
Letter of Credit
Cash Equivalents
Restricted deposits	51
Unsecured Notes
Summary of Significant Accounting Policies [Line Items]
Unsecured notes outstanding	3,700
Unsecured Notes | Matures in 2013
Summary of Significant Accounting Policies [Line Items]
Unsecured notes outstanding	500
Midwest Generation LLC.
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 177

[1] Money market funds are included in cash and cash equivalents and in restricted cash and cash equivalents on EME's consolidated balance sheets.