Cash Equivalents and Investments - Unrealized Loss Position on Fixed Income Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Unrealized Loss
Less than 12 months, fair value	$ 136.9	$ 259.8
Less than 12 months, unrealized loss	(0.2)	(0.2)
12 months or greater, fair value	0.0	11.3
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	136.9	271.1
Total, unrealized loss	(0.2)	(0.2)
Asset-backed securities
Unrealized Loss
Less than 12 months, fair value	10.3	21.6
Less than 12 months, unrealized loss	0.0	0.0
12 months or greater, fair value	0.0	5.2
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	10.3	26.8
Total, unrealized loss	0.0	0.0
Corporate debt securities
Unrealized Loss
Less than 12 months, fair value	105.2	142.6
Less than 12 months, unrealized loss	(0.2)	(0.1)
12 months or greater, fair value	0.0	2.1
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	105.2	144.7
Total, unrealized loss	(0.2)	(0.1)
Foreign government debt securities
Unrealized Loss
Less than 12 months, fair value	8.0	4.0
Less than 12 months, unrealized loss	0.0	0.0
12 months or greater, fair value	0.0	4.0
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	8.0	8.0
Total, unrealized loss	0.0	0.0
U.S. government agency securities
Unrealized Loss
Less than 12 months, fair value		20.0
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		20.0
Total, unrealized loss		0.0
U.S. government securities
Unrealized Loss
Less than 12 months, fair value	13.4	71.6
Less than 12 months, unrealized loss	0.0	(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	13.4	71.6
Total, unrealized loss	$ 0.0	$ (0.1)