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Goodwill and Other Intangibles - Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Goodwill and Other Intangibles      
Goodwill $ 579,000,000 $ 579,000,000 $ 573,000,000
Impairment   2,000,000 6,000,000
Emission allowances held-for-sale 6,000,000 6,000,000 12,000,000
Emission Allowances      
Goodwill and Other Intangibles      
Impairment   0 5,000,000
Impairment related to power purchase agreements     0
BETM | Held-for-sale      
Goodwill and Other Intangibles      
Goodwill 21,000,000 21,000,000  
Goodwill, Impairment Loss 90,000,000    
Midwest Generation      
Goodwill and Other Intangibles      
Goodwill 165,000,000 165,000,000  
Midwest Generation | Lease Agreements      
Goodwill and Other Intangibles      
Out of market contracts     $ 121,000,000
Texas      
Goodwill and Other Intangibles      
Goodwill $ 414,000,000 $ 414,000,000