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Fair Value Measurements - Fair Value of Debt Obligations (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Fair Value | Level 2    
Fair Value of Financial Instruments [Abstract]    
Debt $ 2,260.9 $ 2,275.5
Carrying Value    
Fair Value of Financial Instruments [Abstract]    
Debt $ 2,174.3 $ 2,215.3