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Segment Information - Narrative (Details)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Sep. 30, 2016
USD ($)
Jun. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
segment
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Segment Reporting [Abstract]                      
Number of reportable segments | segment                 1    
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Revenues $ 491.7 $ 457.6 $ 415.8 $ 389.4 $ 405.2 $ 403.6 $ 380.4 $ 365.6 $ 1,754.5 $ 1,554.8 $ 1,678.2
International                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Revenues                 $ 206.4 $ 193.6