Cash Equivalents and Investments - Unrealized Loss Position on Fixed Income Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value
Less than 12 Months	$ 81.4	$ 259.8
12 Months or Greater	0.0	11.3
Total	81.4	271.1
Unrealized Loss
Less than 12 Months	(0.1)	(0.2)
12 Months or Greater	0.0	0.0
Total	(0.1)	(0.2)
Asset-backed securities
Fair Value
Less than 12 Months	17.5	21.6
12 Months or Greater	0.0	5.2
Total	17.5	26.8
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	0.0	0.0
Total	0.0	0.0
Corporate debt securities
Fair Value
Less than 12 Months	61.0	142.6
12 Months or Greater	0.0	2.1
Total	61.0	144.7
Unrealized Loss
Less than 12 Months	(0.1)	(0.1)
12 Months or Greater	0.0	0.0
Total	(0.1)	(0.1)
Foreign government debt securities
Fair Value
Less than 12 Months	0.6	4.0
12 Months or Greater	0.0	4.0
Total	0.6	8.0
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	0.0	0.0
Total	0.0	0.0
U.S. government agency securities
Fair Value
Less than 12 Months		20.0
12 Months or Greater		0.0
Total		20.0
Unrealized Loss
Less than 12 Months		0.0
12 Months or Greater		0.0
Total		0.0
U.S. government securities
Fair Value
Less than 12 Months	2.3	71.6
12 Months or Greater	0.0	0.0
Total	2.3	71.6
Unrealized Loss
Less than 12 Months	0.0	(0.1)
12 Months or Greater	0.0	0.0
Total	$ 0.0	$ (0.1)