Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes	$ 166.8	$ 162.2	$ 133.4
Statutory Federal Income Tax Rate	35.00%	35.00%	35.00%
Income Taxes Computed at 35 Percent Statutory Federal Rate	$ 58.4	$ 56.8	$ 46.7
Increase (Decrease) in Tax [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit	4.4	7.2	4.3
Regulatory Differences for Utility Plant	(0.6)	(3.5)	(2.2)
Production Tax Credits	(37.0)	(23.7)	(19.2)
Other	0.1	(0.1)	(0.9)
Income Tax Expense	$ 25.3	$ 36.7	$ 28.7
Effective Tax Rate on Income	15.20%	22.60%	21.50%