Long-Tern Debt and Capital Lease Obligations - Capital Lease Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Megawatt Number_of_derivative_instruments_held	Dec. 31, 2015 USD ($)
Capital Lease Obligations (Textuals)
Period of power purchse contract with an unaffiliated independent power producer	25 years
Power capacity under capital lease | Megawatt	236
Minimum energy requirement in gas-fired cogeneration facility | Number_of_derivative_instruments_held	0
Power purchase contract expected future renewable period	10 years
repayments of long-term capital lease obligations	$ 37.6	$ 36.2
Increase in regulatory asset due to minimum lease payment	78.5
Regulatory Asset Value At End Of Life Of Contract	0.0
Obligations under capital leases	29.6	59.9
Capital Lease Obligation At End Of Life Of Contract	0.0
Summary of capitalized leased facilities
Long-term power purchase commitment	140.3	140.3
Accumulated amortization	(109.5)	(103.9)
Total leased facilities	30.8	$ 36.4
Future minimum lease payments under capital lease and present value of net minimum lease payments
2017	13.9
2018	14.7
2019	15.5
2020	16.4
2021	17.2
Thereafter	7.6
Total minimum lease payments	85.3
Less: Estimated Executory Costs	(39.9)
Capital Leases Future Minimum Payments Due, Less Executory Costs	45.4
Less: Interest	(15.8)
Present value of net minimum lease payments	29.6
Less: Due currently	(2.7)
Long-term obligations under capital lease	$ 26.9