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Income Taxes (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Income Taxes [Line Items]      
Increase (Decrease) in Effective Tax Rate 0.50% 37.90%  
Deferred Tax Assets, Operating Loss Carryforwards $ 491.0 $ 545.9  
Deferred Tax Assets, Valuation Allowance 7.8 7.8  
Unrecognized Tax Benefits, Income Tax Penalties Expense 0.0 0.0 $ 0.0
Unrecognized Tax Benefits, Income Tax Penalties Accrued $ 0.0 $ 0.0  
Effective Income Tax Rate Reconciliation, Tax Accrual Adjustments, Percent 0.00% 0.00% (28.40%)
Book income (loss) before income taxes $ 956.4 $ 706.6 $ (31.3)
Tax expense (benefit) at statutory federal income tax rate, value $ 200.8 $ 148.3 $ (6.6)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00% 21.00%
State income taxes, net of federal income tax benefit, value $ 4.5 $ 14.1 $ (11.7)
State income taxes, net of federal income tax benefit, rate 0.50% 2.00% 37.40%
Amortization of regulatory liabilities, value $ (38.5) $ (39.1) $ (38.4)
Amortization of regulatory liabilities, rate (4.00%) (5.50%) 122.70%
Fines and penalties, value $ 0.3 $ 0.0 $ 11.8
Fines and penalties, rate 0.00% 0.00% (37.70%)
Effective Income Tax Rate Reconciliation, Deduction, Employee Stock Ownership Plan Dividend, Amount $ (1.2) $ (1.2) $ (1.3)
Employee stock ownership plan dividends and other compensation, rate (0.10%) (0.20%) 4.20%
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act, Amount $ 0.0 $ 0.0 $ 23.3
State regulatory proceedings, rate 0.00% 0.00% (74.50%)
Effective Income Tax Rate Reconciliation Tax Accrual Adjustments $ 0.2 $ (0.1) $ 8.9
Effective Income Tax Rate Reconciliation, Tax Credit, Amount $ (2.3) $ (2.1) $ (2.5)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent (0.20%) (0.30%) (8.00%)
Other adjustments, value $ 0.8 $ (2.1) $ (0.6)
Other adjustments, rate 0.00% (0.30%) 1.90%
Income Taxes $ 164.6 $ 117.8 $ (17.1)
Effective Income Tax Rate Reconciliation, Percent 17.20% 16.70% 54.60%
Massachusetts Business [Domain]      
Income Taxes [Line Items]      
Deferred Tax Assets, Valuation Allowance $ 7.8    
Internal Revenue Service (IRS)      
Income Taxes [Line Items]      
Deferred Tax Assets, Operating Loss Carryforwards 410.0    
State and Local Jurisdiction      
Income Taxes [Line Items]      
Deferred Tax Assets, Operating Loss Carryforwards $ 73.2