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Summary of Significant Accounting Policies - Fair Value Measurements (Details) - Recurring - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Level 2    
Fair Value Measurements    
Interest rate swap agreements and foreign currency forward contracts, gross asset balance $ 1.0  
Interest rate swap agreements and foreign currency forward contracts, gross liability balance 24.8 $ 18.1
Level 3    
Fair Value Measurements    
Investments $ 0.0