Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income [Line Items]
Beginning balance	$ 563.3	$ 249.8	$ 75.9	$ 212.1
Other comprehensive income (loss)	3.0	(1.8)	0.6	(1.7)
Ending balance	595.0	150.6	595.0	150.6
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income [Line Items]
Beginning balance	(4.4)	(3.4)	(1.6)	(2.2)
Other comprehensive income (loss)	3.3	(1.8)	0.5	(3.0)
Ending balance	(1.1)	(5.2)	(1.1)	(5.2)
Unrealized Gain on Available-for-sale Securities
Accumulated Other Comprehensive Income [Line Items]
Beginning balance	0.8	0.6	0.4	(0.7)
Other comprehensive income (loss)	(0.3)	0.0	0.1	1.3
Ending balance	0.5	0.6	0.5	0.6
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income [Line Items]
Beginning balance	(3.6)	(2.8)	(1.2)	(2.9)
Other comprehensive income (loss)	3.0	(1.8)	0.6	(1.7)
Ending balance	$ (0.6)	$ (4.6)	$ (0.6)	$ (4.6)