Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 32.7	$ (17.0)	$ 89.5	$ (46.1)
Other comprehensive income (loss), net of tax:
Change in foreign currency translation adjustments	0.0	0.0	0.0	1.6
Change in net unrealized gains (losses) on short-term investments	(0.6)	0.2	3.5	1.7
Total other comprehensive income (loss), net of tax	(0.6)	0.2	3.5	3.3
Comprehensive income (loss)	$ 32.1	$ (16.8)	$ 93.0	$ (42.8)