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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2015
Dec. 31, 2014
Investments [Abstract]    
Investments $ 1,697 $ 1,393
Equity Method Investments [Abstract]    
Equity method investments 938 868
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 693 657
Investments, including equity method investments and restricted cash and investments 3,328 2,918
Investments, including equity method investments and restricted cash and investments, current 202 115
Investments, including equity method investments and restricted cash and investments, noncurrent 3,126 2,803
Quad Cities Station nuclear decommissioning trust funds [Member]    
Restricted Cash and Investments [Abstract]    
Decommissioning fund investments, fair value 415 424
Solar Star and Topaz Projects [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 125 66
Other restricted cash and investments [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 153 167
ETT [Member]    
Equity Method Investments [Abstract]    
Equity method investments 565 515
Bridger Coal Company [Member]    
Equity Method Investments [Abstract]    
Equity method investments 189 192
Other equity method investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 184 161
BYD Company Limited common stock [Member]    
Investments [Abstract]    
Available-for-sale securities, equity securities 1,186 881
Available-for-sale securities, gross unrealized gain (loss) 954 649
Rabbi trusts [Member]    
Investments [Abstract]    
Rabbi trusts, amount 372 386
Other investments [Member]    
Investments [Abstract]    
Other investments $ 139 $ 126