Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 17.5	$ (21.4)	$ 56.8	$ (29.1)
Other comprehensive income, net of tax:
Change in foreign currency translation adjustments	2.9	(0.3)	(0.3)	1.6
Change in net unrealized gains on short-term investments	6.0	0.4	4.4	1.5
Total other comprehensive income, net of tax	8.9	0.1	4.1	3.1
Comprehensive income (loss)	$ 26.4	$ (21.3)	$ 60.9	$ (26.0)