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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Investments And Restricted Cash And Investments [Line Items]          
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification     $ 0    
Investments [Abstract]          
Investments $ 2,200   2,200   $ 2,526
Equity Method Investments [Abstract]          
Equity method investments 2,030   2,030   1,834
Restricted Cash and Investments [Abstract]          
Restricted cash and investments 928   928   863
Investments, including equity method investments and restricted cash and investments 5,158   5,158   5,223
Investments, including equity method investments and restricted cash and investments, current 404   404   351
Investments, including equity method investments and restricted cash and investments, noncurrent 4,754   4,754   4,872
Gain on Sale of Investments 260 $ 3 (336) $ 8  
Marketable Securities, Realized Gain (Loss) 0   1    
Marketable Securities, Gain (Loss) 260   (337)    
Quad Cities Station nuclear decommissioning trust funds [Member]          
Restricted Cash and Investments [Abstract]          
Decommissioning fund investments, fair value 543   543   515
Other restricted cash and investments [Member]          
Restricted Cash and Investments [Abstract]          
Restricted cash and investments 385   385   348
Equity Method Investments, tax equity investments [Member]          
Equity Method Investments [Abstract]          
Equity method investments 1,221   1,221   1,025
ETT [Member]          
Equity Method Investments [Abstract]          
Equity method investments 530   530   524
Bridger Coal Company [Member]          
Equity Method Investments [Abstract]          
Equity method investments 116   116   137
Other equity method investments [Member]          
Equity Method Investments [Abstract]          
Equity method investments 163   163   148
BYD Company Limited common stock [Member]          
Investments [Abstract]          
Available-for-sale securities, equity securities 1,616   1,616   1,961
Rabbi trusts [Member]          
Investments [Abstract]          
Rabbi trusts, amount 398   398   441
Other investments [Member]          
Investments [Abstract]          
Other investments $ 186   186   $ 124
Retained Earnings [Member]          
Investments And Restricted Cash And Investments [Line Items]          
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification     $ 1,085