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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Investments [Abstract]    
Investments $ 1,936 $ 1,694
Equity Method Investments [Abstract]    
Equity method investments 1,798 1,720
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 723 742
Investments, including equity method investments and restricted cash and investments 4,457 4,156
Investments, including equity method investments and restricted cash and investments, current 196 211
Investments, including equity method investments and restricted cash and investments, noncurrent 4,261 3,945
Quad Cities Station nuclear decommissioning trust funds [Member]    
Restricted Cash and Investments [Abstract]    
Decommissioning fund investments, fair value 485 460
Other restricted cash and investments [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 238 282
Equity Method Investments, tax equity investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 811 741
ETT [Member]    
Equity Method Investments [Abstract]    
Equity method investments 694 672
Bridger Coal Company [Member]    
Equity Method Investments [Abstract]    
Equity method investments 150 165
Other equity method investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 143 142
BYD Company Limited common stock [Member]    
Investments [Abstract]    
Available-for-sale securities, equity securities 1,381 1,185
Available-for-sale securities, gross unrealized gain (loss) 1,149 953
Rabbi trusts [Member]    
Investments [Abstract]    
Rabbi trusts, amount 427 403
Other investments [Member]    
Investments [Abstract]    
Other investments $ 128 $ 106