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Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance $ 77 $ 32
Amounts Incurred 33 [1] 16 [2]
Amounts Settled (2) [3] (2) [4]
Accretion 4 2
Cash flow revisions [5]   (29)
Ending balance 112 [6] 77
Electric Plant Steam Production Asbestos    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance 51 22
Amounts Incurred 0 [1] 0 [2]
Amounts Settled (2) [3] (2) [4]
Accretion 3 1
Cash flow revisions [5]   (30)
Ending balance 52 [6] 51
Electric Plant Wind Production [Member]    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance 16 0
Amounts Incurred 33 [1] 16 [2]
Amounts Settled 0 [3] 0 [4]
Accretion 1 0
Cash flow revisions [5]   0
Ending balance 50 [6] 16
Electric Plant Distribution    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance 10 9
Amounts Incurred 0 [1] 0 [2]
Amounts Settled 0 [3] 0 [4]
Accretion 0 1
Cash flow revisions [5]   0
Ending balance 10 [6] 10
Electric Plant Other Sources    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance $ 0 1
Amounts Incurred [2]   0
Amounts Settled [4]   0
Accretion   0
Cash flow revisions [5]   (1)
Ending balance   $ 0
[1] Amounts incurred related to the Sagamore wind farm placed in service in 2020.
[2] Amounts incurred related to the Hale wind farm placed in service in 2019.
[3] Amounts settled related mainly to asbestos abatement projects.
[4] Amounts settled related to asbestos abatement projects.
[5] In 2019, AROs were revised for changes in timing and estimates of cash flows. Changes in steam production AROs primarily related to the cost estimates to remediate ponds at production facilities.
[6] No AROs were revised in 2020.