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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Investments [Abstract]      
Investments $ 1,974 $ 2,526  
Gain (Loss) on Investments (538) 14 $ 10
Marketable Securities, Realized Gain (Loss) 2    
Marketable Securities, Unrealized Gain (Loss) (540)    
Equity Method Investments [Abstract]      
Tax Equity Contributions 698 403 $ 584
Equity method investments 2,440 1,834  
Restricted Cash and Investments [Abstract]      
Restricted cash and investments 760 863  
Investments, including equity method and restricted cash and investments 5,174 5,223  
Investments, including equity method and restricted cash and investments, noncurrent 4,903 4,872  
Investments, including equity method and restricted cash and investments, current 271 351  
Quad Cities Station nuclear decommissioning trust funds [Member]      
Restricted Cash and Investments [Abstract]      
Decommissioning Fund Investments, Fair Value 504 515  
Restricted Cash and Investments, Other [Member]      
Restricted Cash and Investments [Abstract]      
Restricted cash and investments 256 348  
Electric Transmission Texas, LLC [Member]      
Equity Method Investments [Abstract]      
Equity method investments $ 527 524  
Equity method investment, ownership percentage 50.00%    
Bridger Coal Company [Member]      
Equity Method Investments [Abstract]      
Equity method investments $ 99 137  
Equity method investment, ownership percentage 66.67%    
Tax Equity Investments      
Equity Method Investments [Abstract]      
Equity method investments $ 1,661 1,025  
Other equity method investments [Member]      
Equity Method Investments [Abstract]      
Equity method investments 153 148  
BYD Company Limited common stock [Member]      
Investments [Abstract]      
Available-for-sale Securities, Equity Securities 1,435 1,961  
Rabbi trusts [Member]      
Investments [Abstract]      
Life Insurance, Corporate or Bank Owned, Amount 371 441  
Other investments [Member]      
Investments [Abstract]      
Other Investments $ 168 $ 124