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Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Carrying amounts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of long-term debt $ 159.6 $ 163.9
Fair value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of long-term debt $ 139.8 $ 114.8