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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Investments [Abstract]      
Investments $ 6,600 $ 1,719  
Gain (Loss) on Investments 4,797 (288) $ (538)
Marketable Securities, Realized Gain (Loss) 6 2  
Marketable Securities, Unrealized Gain (Loss) 4,791 (290)  
Equity method investments:      
Tax Equity Contributions 2,736 1,619 $ 698
Purchase Obligation, Due in Next Twelve Months 563    
Equity method investments 7,067 3,947  
Restricted cash and cash equivalents:      
Restricted cash and investments 831 829  
Investments, including equity method and restricted cash and investments 14,498 6,495  
Investments, including equity method and restricted cash and investments, noncurrent 14,320 6,255  
Investments, including equity method and restricted cash and investments, current 178 240  
Eastern Energy Gas Holdings, LLC [Member]      
Equity method investments:      
Equity method investments 244 312  
Restricted cash and cash equivalents:      
Restricted cash and investments 13 12  
Investments, including equity method and restricted cash and investments 257 324  
Investments, including equity method and restricted cash and investments, noncurrent 244 312  
Investments, including equity method and restricted cash and investments, current 13 12  
Quad Cities Station nuclear decommissioning trust funds [Member]      
Restricted cash and cash equivalents:      
Decommissioning Fund Investments, Fair Value 676 599  
Restricted Cash and Investments, Other [Member]      
Restricted cash and cash equivalents:      
Restricted cash and investments 155 230  
Electric Transmission Texas, LLC [Member]      
Equity method investments:      
Equity method investments $ 594 555  
Equity method investment, ownership percentage 50.00%    
Bridger Coal Company [Member]      
Equity method investments:      
Equity method investments $ 74 81  
Equity method investment, ownership percentage 66.67%    
Tax Equity Investments      
Equity method investments:      
Equity method investments $ 5,626 3,130  
Other equity method investments [Member]      
Equity method investments:      
Equity method investments 118 181  
Iroquois Gas Transmission System L.P. [Member]      
Equity method investments:      
Equity method investments $ 580 0  
Equity method investment, ownership percentage 50.00%    
Iroquois Gas Transmission System L.P. [Member] | Eastern Energy Gas Holdings, LLC [Member]      
Equity method investments:      
Equity method investments $ 244 276  
Equity method investment, ownership percentage 50.00%    
Equity Method Investments, JAX LNG, LLC      
Equity method investments:      
Equity method investments $ 75 0  
Equity method investment, ownership percentage 50.00%    
BYD Company Limited common stock [Member]      
Investments [Abstract]      
Available-for-sale Securities, Equity Securities $ 5,897 1,122  
Rabbi trusts [Member]      
Investments [Abstract]      
Life Insurance, Corporate or Bank Owned, Amount 440 410  
Other investments [Member]      
Investments [Abstract]      
Other Investments $ 263 $ 187