Cash Equivalents and Investments - Unrealized Loss Position on Fixed Income Securities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Unrealized Loss
Less than 12 months, fair value	$ 202.0	$ 259.8
Less than 12 months, unrealized loss	(0.5)	(0.2)
12 months or greater, fair value	4.0	11.3
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	206.0	271.1
Total, unrealizeud loss	(0.5)	(0.2)
Asset-backed securities
Unrealized Loss
Less than 12 months, fair value	9.2	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	9.2	26.8
Total, unrealizeud loss	0.0	0.0
Corporate debt securities
Unrealized Loss
Less than 12 months, fair value	167.8	142.6
Less than 12 months, unrealized loss	(0.5)	(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	167.8	144.7
Total, unrealizeud loss	(0.5)	(0.1)
Foreign government debt securities
Unrealized Loss
Less than 12 months, fair value	4.0	4.0
Less than 12 months, unrealized loss	0.0	0.0
12 months or greater, fair value	4.0	4.0
12 months or greater, unrealized loss	0.0	0.0
Total, fair value	8.0	8.0
Total, unrealizeud loss	0.0	0.0
U.S. government agency securities
Unrealized Loss
Less than 12 months, fair value	6.0	20.0
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	6.0	20.0
Total, unrealizeud loss	0.0	0.0
U.S. government securities
Unrealized Loss
Less than 12 months, fair value	15.0	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	15.0	71.6
Total, unrealizeud loss	$ 0.0	$ (0.1)