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Income Taxes - MEC - Reconciliation of Effective Income Tax Rate (Details)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Schedule of Effective Income Tax Rate Reconciliation [Line Items]      
Federal statutory income tax rate 21.00% 21.00% 21.00%
Income tax credits (27.00%) (16.00%) (32.00%)
State income tax, net of federal income tax benefit (10.00%) 3.00% (5.00%)
Effects of ratemaking (4.00%) (3.00%) (6.00%)
Other, net 0.00% (1.00%) (1.00%)
Effective income tax rate (21.00%) 4.00% (25.00%)
Years eligible for renewable energy production tax credit 10 years    
MEC      
Schedule of Effective Income Tax Rate Reconciliation [Line Items]      
Federal statutory income tax rate 21.00% 21.00% 21.00%
Income tax credits (262.00%) (199.00%) (90.00%)
State income tax, net of federal income tax benefit (46.00%) (27.00%) (11.00%)
Effects of ratemaking (20.00%) (17.00%) (8.00%)
Other, net (1.00%) (1.00%) 0.00%
Effective income tax rate (308.00%) (223.00%) (88.00%)
Years eligible for renewable energy production tax credit 10 years