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Summary of Significant Accounting Policies (Property, Plant and Equipment) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Property, Plant and Equipment [Line Items]      
Interest capitalized $ 7 $ 31 $ 27
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 [Member]
     
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 [Member]
     
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 [Member]
     
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 [Member]
     
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 [Member]
     
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 [Member]
     
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 [Member]
     
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 [Member]
     
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, useful lives (in years) 33 years 9 months    
Joliet Unit 6 | Midwest Generation LLC [Member]
     
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, useful lives (in years) 5 years    
Joliet Units 7 and 8 | Midwest Generation LLC [Member]
     
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, useful lives (in years) 17 years [1]    
Powerton Station | Midwest Generation LLC [Member]
     
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, useful lives (in years) 20 years [1]    
[1] Represents leased facilities. The leases may be renewed based on criteria outlined in their respective agreements.