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Fair Value - Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Fair Value of Financial Instruments [Line Items]    
Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount $ 1,519.0 $ 1,569.1
Level 2 [Member]    
Fair Value of Financial Instruments [Line Items]    
Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value $ 1,627.9 $ 1,653.8