Cash Equivalents and Investments - Unrealized Loss Position on Fixed Income Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Unrealized Loss
Less than 12 months, fair value	$ 139.1	$ 81.4
Less than 12 months, unrealized loss	(0.2)	(0.1)
12 months or greater, fair value	0.0	0.0
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	139.1	81.4
Total, unrealized loss	(0.2)	(0.1)
Asset-backed securities
Unrealized Loss
Less than 12 months, fair value	31.1	17.5
Less than 12 months, unrealized loss	0.0	0.0
12 months or greater, fair value	0.0	0.0
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	31.1	17.5
Total, unrealized loss	0.0	0.0
Commercial paper
Unrealized Loss
Less than 12 months, fair value	1.5
Less than 12 months, unrealized loss	0.0
12 months or greater, fair value	0.0
12 months or greater, unrealized loss	0.0
Total, fair value	1.5
Total, unrealized loss	0.0
Corporate debt securities
Unrealized Loss
Less than 12 months, fair value	91.8	61.0
Less than 12 months, unrealized loss	(0.2)	(0.1)
12 months or greater, fair value	0.0	0.0
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	91.8	61.0
Total, unrealized loss	(0.2)	(0.1)
Foreign government debt securities
Unrealized Loss
Less than 12 months, fair value		0.6
Less than 12 months, unrealized loss		0.0
12 months or greater, fair value		0.0
12 months or greater, unrealized loss		0.0
Total, fair value		0.6
Total, unrealized loss		0.0
U.S. government securities
Unrealized Loss
Less than 12 months, fair value	14.7	2.3
Less than 12 months, unrealized loss	0.0	0.0
12 months or greater, fair value	0.0	0.0
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	14.7	2.3
Total, unrealized loss	$ 0.0	$ 0.0