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Income Taxes (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Taxes [Line Items] |  |  |  |
Tax expense at statutory federal income tax rate, rate |  | 21.00% | 35.00% | 35.00%
Effective income tax rate |  | 78.10% | 71.00% | 35.50%
Increase (Decrease) in Effective Tax Rate |  | 7.10% | 35.50% |
State regulatory proceedings, value |  | $ 127.8 | $ 0.0 | $ 0.0
Increase in State Income Taxes |  | 7.1 |  |
Tax Benefit From Excess Deferred Tax Amortization Related To Regulatory LIabilities |  | 26.9 |  |
Expected benefits to be realized | $ 65.8 | $ 88.5 | $ 65.8 |
Income Tax Rate Change Period |  |  | 6 years |
Operating Loss Carryforwards, Limitations on Use |  | 80.00% |  |
Operating Loss Carryforwards, Valuation Allowance |  | $ 759.6 |  |
Operating Loss Carryforwards, Valuation Allowance, years prior to TCJA |  | 508.5 |  |
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax |  | 1.7 |  |
Unrecognized Tax Benefits, Income Tax Penalties Accrued | 0.0 | 0.0 | $ 0.0 | 0.0
Reduction in Deferred Tax Liability Due To Tax Law Change |  |  | 1,300.0 |
Reduction in Regulatory Deferred tax Liabilities |  |  | 400.0 |
Increase in Regulatory Liability Due to Tax Law Change |  |  | 1,500.0 |
Increase in Tax Expense Due to Tax Law Change |  |  | 200.0 |
Remeasurement due to TCJA, value | $ 161.1 | $ 0.0 | $ 161.1 | $ 0.0
Minimum |  |  |  |
Income Taxes [Line Items] |  |  |  |
Tax Credit Carryforward, Expiration Date |  | Dec. 31, 2028 |  |
Maximum |  |  |  |
Income Taxes [Line Items] |  |  |  |
Tax Credit Carryforward, Expiration Date |  | Dec. 31, 2037 |  |
Federal tax on excess stock compensation [Domain] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Change in tax expense |  | $ 120.7 |  |

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