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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Investments [Abstract]      
Investments $ 1,930   $ 1,974
Equity Method Investments [Abstract]      
Equity Method Investments 2,411   2,440
Restricted Cash and Investments [Abstract]      
Restricted cash and investments 741   760
Investments, including equity method investments and restricted cash and investments 5,082   5,174
Investments, including equity method investments and restricted cash and investments, current 205   271
Investments, including equity method investments and restricted cash and investments, noncurrent 4,877   4,903
Gain on Sale of Investments (68) $ (209)  
Marketable Securities, Realized Gain (Loss) 0 2  
Marketable Securities, Gain (Loss) (68) (211)  
Quad Cities Station nuclear decommissioning trust funds [Member]      
Restricted Cash and Investments [Abstract]      
Decommissioning fund investments, fair value 545   504
Other restricted cash and investments [Member]      
Restricted Cash and Investments [Abstract]      
Restricted cash and investments 196   256
Equity Method Investments, tax equity investments [Member]      
Equity Method Investments [Abstract]      
Equity Method Investments 1,627   1,661
ETT [Member]      
Equity Method Investments [Abstract]      
Equity Method Investments 529   527
Bridger Coal Company [Member]      
Equity Method Investments [Abstract]      
Equity Method Investments 97   99
Other equity method investments [Member]      
Equity Method Investments [Abstract]      
Equity Method Investments 158   153
BYD Company Limited common stock [Member]      
Investments [Abstract]      
Available-for-sale securities, equity securities 1,356   1,435
Rabbi trusts [Member]      
Investments [Abstract]      
Rabbi trusts, amount 394   371
Other investments [Member]      
Investments [Abstract]      
Other investments $ 180   $ 168
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