Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income [Line Items]
Beginning balance	$ 59.0	$ 223.6	$ 75.9	$ 212.1
Other comprehensive income	0.2	0.3	(2.4)	0.1
Ending balance	563.3	249.8	563.3	249.8
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income [Line Items]
Beginning balance	(5.0)	(3.0)	(1.6)	(2.2)
Other comprehensive income	0.6	(0.4)	(2.8)	(1.2)
Ending balance	(4.4)	(3.4)	(4.4)	(3.4)
Unrealized Gain on Available-for-sale Securities
Accumulated Other Comprehensive Income [Line Items]
Beginning balance	1.2	(0.1)	0.4	(0.7)
Other comprehensive income	(0.4)	0.7	0.4	1.3
Ending balance	0.8	0.6	0.8	0.6
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income [Line Items]
Beginning balance	(3.8)	(3.1)	(1.2)	(2.9)
Other comprehensive income	0.2	0.3	(2.4)	0.1
Ending balance	$ (3.6)	$ (2.8)	$ (3.6)	$ (2.8)