Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance as of December 31, 2017			$ 313.1
Change in other comprehensive loss	$ 0.1	$ 0.1	(0.4)	$ 0.2
Balance as of September 30, 2018	358.7		358.7
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			(0.4)
Balance as of September 30, 2018	(1.6)		(1.6)
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 September 30, 2018	$ (1.6)		$ (1.6)