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Fair Value - Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Fair Value of Financial Instruments [Line Items]    
Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount $ 1,528.8 $ 1,513.3
Level 2 [Member]    
Fair Value of Financial Instruments [Line Items]    
Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value $ 1,572.8 $ 1,627.6