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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Investments [Abstract]    
Investments $ 1,694 $ 1,748
Equity Method Investments [Abstract]    
Equity method investments 1,720 1,103
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 742 653
Investments, including equity method and restricted cash and investments, current 211 137
Investments, including equity method and restricted cash and investments, noncurrent 3,945 3,367
Investments, including equity method and restricted cash and investments 4,156 3,504
Quad Cities Station nuclear decommissioning trust funds [Member]    
Restricted Cash and Investments [Abstract]    
Decommissioning Fund Investments, Fair Value 460 429
Solar Star and Topaz Projects [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 64 95
Restricted Cash and Investments, Other [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 218 129
Electric Transmission Texas, LLC [Member]    
Equity Method Investments [Abstract]    
Equity method investments $ 672 585
Equity method investment, ownership percentage 50.00%  
Bridger Coal Company [Member]    
Equity Method Investments [Abstract]    
Equity method investments $ 165 190
Equity method investment, ownership percentage 66.67%  
Tax Equity Investments    
Equity Method Investments [Abstract]    
Equity method investments $ 741 168
Other equity method investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 142 160
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 953 1,006
Investments [Abstract]    
Available-for-sale Securities, Equity Securities 1,185 1,238
Rabbi trusts [Member]    
Investments [Abstract]    
Life Insurance, Corporate or Bank Owned, Amount 403 380
Other investments [Member]    
Investments [Abstract]    
Other Investments $ 106 $ 130