Summary of Significant Accounting Policies (Property, Plant and Equipment) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Interest capitalized	$ 31		$ 27	$ 54
Power plant facilities | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	2 years 9 months
Power plant facilities | Minimum | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	2 years 9 months
Power plant facilities | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	35 years
Power plant facilities | Maximum | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	30 years
Emission allowances | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	25 years
Emission allowances | Minimum | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	25 years
Emission allowances | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	33 years 9 months
Emission allowances | Maximum | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	33 years 9 months
Equipment, furniture and fixtures | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	3 years
Equipment, furniture and fixtures | Minimum | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	3 years
Equipment, furniture and fixtures | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	10 years
Equipment, furniture and fixtures | Maximum | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	7 years
Plant and equipment under lease financing | Minimum | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	30 years
Plant and equipment under lease financing | Maximum | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	33 years 9 months
Joliet Unit 6 | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	6 years
Joliet Units 7 and 8 | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	18 years	[1]
Powerton Station | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	21 years	[1]
Will County Station | Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful lives (in years)	17 years

[1] Represents leased facilities. The leases may be renewed based on criteria outlined in their respective agreements.