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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Investments [Abstract]    
Investments $ 2,650 $ 1,694
Equity Method Investments [Abstract]    
Equity method investments 1,795 1,720
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 815 742
Investments, including equity method investments and restricted cash and investments 5,260 4,156
Investments, including equity method investments and restricted cash and investments, current 273 211
Investments, including equity method investments and restricted cash and investments, noncurrent 4,987 3,945
Quad Cities Station nuclear decommissioning trust funds [Member]    
Restricted Cash and Investments [Abstract]    
Decommissioning fund investments, fair value 498 460
Other restricted cash and investments [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 317 282
Equity Method Investments, tax equity investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 804 741
ETT [Member]    
Equity Method Investments [Abstract]    
Equity method investments 693 672
Bridger Coal Company [Member]    
Equity Method Investments [Abstract]    
Equity method investments 140 165
Other equity method investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 158 142
BYD Company Limited common stock [Member]    
Investments [Abstract]    
Available-for-sale securities, equity securities 2,087 1,185
Available-for-sale securities, gross unrealized gain (loss) 1,855 953
Rabbi trusts [Member]    
Investments [Abstract]    
Rabbi trusts, amount 431 403
Other investments [Member]    
Investments [Abstract]    
Other investments $ 132 $ 106