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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
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]      
Income Before Non-Controlling Interest and Income Taxes $ 186.9 $ 175.6 $ 166.8
Statutory Federal Income Tax Rate 35.00% 35.00% 35.00%
Income Taxes Computed at 35 Percent Statutory Federal Rate $ 65.4 $ 61.5 $ 58.4
Increase (Decrease) in Tax Due to: [Abstract]      
State Income Taxes – Net of Federal Income Tax Benefit 10.5 5.6 4.4
Regulatory Differences for Utility Plant 0.0 (0.1) (0.6)
Production Tax Credits (45.1) (41.5) (37.0)
Change in Fair Value of Contingent Consideration 0.0 (3.8) 0.0
Remeasurement of Deferred Income Taxes [1] (13.0) 0.0 0.0
Other (3.1) (1.9) 0.1
Total Income Tax Expense $ 14.7 $ 19.8 $ 25.3
Effective Tax Rate 7.90% 11.30% 15.20%
[1] Deferred income tax benefit from the remeasurement of deferred income tax assets and liabilities resulting from the TCJA. (See Note 1. Operations and Significant Accounting Policies.)