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Segment Reporting - Interests (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
item
Interests    
Number of major interests | item 2 3
Revenue $ 675.0 $ 610.2
Major Interest One    
Interests    
Revenue $ 105.2 $ 92.5
Percentage of revenue (as a percent) 15.60% 15.20%
Major Interest Two    
Interests    
Revenue $ 90.2 $ 88.5
Percentage of revenue (as a percent) 13.40% 14.50%
Major Interest Three    
Interests    
Revenue   $ 75.0
Percentage of revenue (as a percent)   12.30%