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Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] |  |  |  |
Income Before Non-Controlling Interest and Income Taxes |  | $ 158.6 | $ 186.9 | $ 175.6
Statutory Federal Income Tax Rate |  | 21.00% | 35.00% | 35.00%
Income Taxes Computed at Statutory Federal Rate |  | $ 33.3 | $ 65.4 | $ 61.5
Increase (Decrease) in Tax Due to: [Abstract] |  |  |  |
State Income Taxes – Net of Federal Income Tax Benefit |  | 8.9 | 10.5 | 5.6
Production Tax Credits |  | (45.0) | (45.1) | (41.5)
Regulatory Differences – Excess Deferred Tax Benefit | [1] | (8.2) | 1.2 | 1.4
Change in Fair Value of Contingent Consideration |  | (0.4) | 0.0 | (3.8)
Remeasurement of Deferred Income Taxes | [2] | 0.0 | (13.0) | 0.0
Other |  | (4.1) | (4.3) | (3.4)
Total Income Tax Expense (Benefit) |  | $ (15.5) | $ 14.7 | $ 19.8
Effective Tax Rate |  | 9.80% | 7.90% | 11.30%
[1] | Excess deferred income taxes are being returned to customers under both the Average Rate Assumption Method and amortization periods as approved by regulators. (See Note 4. Regulatory Matters.)
[2] | Deferred income tax benefit from the remeasurement of deferred income tax assets and liabilities resulting from the TCJA.

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