Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance as of December 31, 2018	$ 368.6	$ 313.1
Change in other comprehensive loss	0.2	(0.4)
Balance as of March 31, 2019	377.0	328.3
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance as of December 31, 2018	(2.4)
Change in other comprehensive loss	0.2
Balance as of March 31, 2019	(2.2)
Total Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance as of December 31, 2018	(2.4)	(1.2)
Balance as of March 31, 2019	$ (2.2)	$ (1.6)