Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance as of December 31, 2017			$ 313.1
Change in other comprehensive loss	$ (0.2)	$ 0.1	(0.6)	$ 0.1
Balance as of June 30, 2018	345.2		345.2
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance as of December 31, 2017			(1.2)
Change in other comprehensive loss			(0.6)
Balance as of June 30, 2018	(1.8)		(1.8)
Total Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance as of December 31, 2017			(1.2)
Balance as of June 30, 2018	$ (1.8)		$ (1.8)