Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Cumulative Effect of New Accounting Principle in Period of Adoption       $ (176)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]        
Accumulated Other Comprehensive Income (Loss), Beginning Balance $ (229) $ (263) $ (263)  
Other Comprehensive Income before Reclassifications (16) 30    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 10 (9)    
Other Comprehensive Income (Loss), net of tax (6) 21    
Accumulated Other Comprehensive Income (Loss), Ending Balance (411) (242) (229)  
Net Change in Accumulated Other Comprehensive Income (182)      
Cash Flow Hedges [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Cumulative Effect of New Accounting Principle in Period of Adoption       0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]        
Accumulated Other Comprehensive Income (Loss), Beginning Balance 0 2 2  
Other Comprehensive Income before Reclassifications 0 0 0  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 0 0 (2)  
Other Comprehensive Income (Loss), net of tax 0 0    
Accumulated Other Comprehensive Income (Loss), Ending Balance 0 2 0  
Net Change in Accumulated Other Comprehensive Income 0      
Pension and OPEB Plans [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Cumulative Effect of New Accounting Principle in Period of Adoption       0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]        
Accumulated Other Comprehensive Income (Loss), Beginning Balance (406) (398) (398)  
Other Comprehensive Income before Reclassifications 0 0    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 8 6    
Other Comprehensive Income (Loss), net of tax 8 6    
Accumulated Other Comprehensive Income (Loss), Ending Balance (398) (392) (406)  
Net Change in Accumulated Other Comprehensive Income 8      
Available-for-Sale Securities [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Cumulative Effect of New Accounting Principle in Period of Adoption       (176)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]        
Accumulated Other Comprehensive Income (Loss), Beginning Balance 177 133 133  
Other Comprehensive Income before Reclassifications (16) 30    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 2 (15)    
Other Comprehensive Income (Loss), net of tax (14) 15    
Accumulated Other Comprehensive Income (Loss), Ending Balance (13) 148 177  
Net Change in Accumulated Other Comprehensive Income (190)      
Power [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Cumulative Effect of New Accounting Principle in Period of Adoption       (175)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]        
Accumulated Other Comprehensive Income (Loss), Beginning Balance (172) (211) (211)  
Other Comprehensive Income before Reclassifications (13) 28    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 8 (4)    
Other Comprehensive Income (Loss), net of tax (5) 24    
Accumulated Other Comprehensive Income (Loss), Ending Balance (352) (187) (172)  
Net Change in Accumulated Other Comprehensive Income (180)      
Power [Member] | Cash Flow Hedges [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Cumulative Effect of New Accounting Principle in Period of Adoption       0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]        
Accumulated Other Comprehensive Income (Loss), Beginning Balance 0 0 0  
Other Comprehensive Income before Reclassifications 0 0    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 0 0    
Other Comprehensive Income (Loss), net of tax 0 0    
Accumulated Other Comprehensive Income (Loss), Ending Balance 0 0 0  
Net Change in Accumulated Other Comprehensive Income 0      
Power [Member] | Pension and OPEB Plans [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Cumulative Effect of New Accounting Principle in Period of Adoption       0
Accumulated Other Comprehensive Income (Loss) [Roll Forward]        
Accumulated Other Comprehensive Income (Loss), Beginning Balance (347) (340) (340)  
Other Comprehensive Income before Reclassifications 0 0    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 6 5    
Other Comprehensive Income (Loss), net of tax 6 5    
Accumulated Other Comprehensive Income (Loss), Ending Balance (341) (335) (347)  
Net Change in Accumulated Other Comprehensive Income 6      
Power [Member] | Available-for-Sale Securities [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Cumulative Effect of New Accounting Principle in Period of Adoption       $ (175)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]        
Accumulated Other Comprehensive Income (Loss), Beginning Balance 175 129 129  
Other Comprehensive Income before Reclassifications (13) 28    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 2 (9)    
Other Comprehensive Income (Loss), net of tax (11) 19    
Accumulated Other Comprehensive Income (Loss), Ending Balance (11) $ 148 $ 175  
Net Change in Accumulated Other Comprehensive Income $ (186)