Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes	$ 162.2	$ 133.4	$ 135.1
Statutory Federal Income Tax Rate	35.00%	35.00%	35.00%
Income Taxes Computed at 35 Percent Statutory Federal Rate	56.8	46.7	47.3
Increase (Decrease) in Tax [Abstract]
State Income Taxes - Net of Federal Income Tax Benefit	7.2	4.3	1.2
Regulatory Differences for Utility Plant	(3.5)	(2.2)	(2.2)
Production Tax Credits	(23.7)	(19.2)	(7.6)
Other	(0.1)	(0.9)	(0.7)
Income Tax Expense	$ 36.7	$ 28.7	$ 38.0
Effective Income Tax Rate	22.60%	21.50%	28.10%