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Long-Tern Debt and Capital Lease Obligations - Capital Lease Obligations (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Megawatt
Number_of_derivative_instruments_held
Dec. 31, 2016
USD ($)
Capital Lease Obligations (Textuals)    
Period of power purchase 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 $ 7.2 $ 37.6
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 27.0 29.6
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 (115.2) (109.5)
Total leased facilities 25.1 $ 30.8
Future minimum lease payments under capital lease and present value of net minimum lease payments    
2018 14.7  
2019 15.5  
2020 16.4  
2021 17.2  
2022 7.6  
Thereafter 0.0  
Total minimum lease payments 71.4  
Less: Estimated executory costs (33.1)  
Net minimum lease payments 38.3  
Less: Interest (11.3)  
Present value of net minimum lease payments 27.0  
Less: Due currently (3.7)  
Long-term obligations under capital lease $ 23.3