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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
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 2,324 $ 2,526
Equity Method Investments [Abstract]    
Equity method investments 1,985 1,834
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 744 863
Investments, including equity method investments and restricted cash and investments 5,053 5,223
Investments, including equity method investments and restricted cash and investments, current 257 351
Investments, including equity method investments and restricted cash and investments, noncurrent 4,796 4,872
Gain on Sale of Investments (209)  
Marketable Securities, Realized Gain (Loss) 2  
Marketable Securities, Gain (Loss) (211)  
Quad Cities Station nuclear decommissioning trust funds [Member]    
Restricted Cash and Investments [Abstract]    
Decommissioning fund investments, fair value 512 515
Other restricted cash and investments [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 232 348
Equity Method Investments, tax equity investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 1,177 1,025
ETT [Member]    
Equity Method Investments [Abstract]    
Equity method investments 536 524
Bridger Coal Company [Member]    
Equity Method Investments [Abstract]    
Equity method investments 129 137
Other equity method investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 143 148
BYD Company Limited common stock [Member]    
Investments [Abstract]    
Available-for-sale securities, equity securities 1,754 1,961
Rabbi trusts [Member]    
Investments [Abstract]    
Rabbi trusts, amount 387 441
Other investments [Member]    
Investments [Abstract]    
Other investments 183 $ 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