Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes	$ 175.6	$ 166.8	$ 162.2
Statutory Federal Income Tax Rate	35.00%	35.00%	35.00%
Income Taxes Computed at 35 Percent Statutory Federal Rate	$ 61.5	$ 58.4	$ 56.8
Increase (Decrease) in Tax Due to: [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit	5.6	4.4	7.2
Regulatory Differences for Utility Plant	(0.1)	(0.6)	(3.5)
Production Tax Credits	(41.5)	(37.0)	(23.7)
Change in Fair Value of Contingent Consideration	(3.8)	0.0	0.0
Other	(1.9)	0.1	(0.1)
Total Income Tax Expense	$ 19.8	$ 25.3	$ 36.7
Effective Tax Rate	11.30%	15.20%	22.60%