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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 |  |

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