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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments [Abstract]      
Investments $ 2,526 $ 1,694  
Equity Method Investments [Abstract]      
Tax Equity Contributions 403 584 $ 170
Equity method investments 1,834 1,720  
Restricted Cash and Investments [Abstract]      
Restricted cash and investments 863 742  
Investments, including equity method and restricted cash and investments, current 351 211  
Investments, including equity method and restricted cash and investments, noncurrent 4,872 3,945  
Investments, including equity method and restricted cash and investments 5,223 4,156  
Quad Cities Station nuclear decommissioning trust funds [Member]      
Restricted Cash and Investments [Abstract]      
Decommissioning Fund Investments, Fair Value 515 460  
Restricted Cash and Investments, Other [Member]      
Restricted Cash and Investments [Abstract]      
Restricted cash and investments 348 282  
Electric Transmission Texas, LLC [Member]      
Equity Method Investments [Abstract]      
Equity method investments $ 524 672  
Equity method investment, ownership percentage 50.00%    
Bridger Coal Company [Member]      
Equity Method Investments [Abstract]      
Equity method investments $ 137 165  
Equity method investment, ownership percentage 66.67%    
Tax Equity Investments      
Equity Method Investments [Abstract]      
Equity method investments $ 1,025 741  
Other equity method investments [Member]      
Equity Method Investments [Abstract]      
Equity method investments 148 142  
BYD Company Limited common stock [Member]      
Investments, Including Equity Method And Restricted Cash And Investments [Line Items]      
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss) before Taxes 1,729 953  
Investments [Abstract]      
Available-for-sale Securities, Equity Securities 1,961 1,185  
Rabbi trusts [Member]      
Investments [Abstract]      
Life Insurance, Corporate or Bank Owned, Amount 441 403  
Other investments [Member]      
Investments [Abstract]      
Other Investments $ 124 $ 106