Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance as of December 31, 2017	$ 313.1
Change in other comprehensive loss	(0.4)	$ (0.1)
Balance as of March 31, 2018	328.3
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 March 31, 2018	(1.6)
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 March 31, 2018	$ (1.6)