Cash Equivalents and Investments - Unrealized Loss Position on Fixed Income Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 Months	$ 259.8	$ 423.2
12 Months or Greater	11.3	500.8
Total	271.1	924.0
Unrealized Loss
Less than 12 Months	(0.2)	(0.1)
12 Months or Greater	0.0	(4.1)
Total	(0.2)	(4.2)
Asset-backed securities
Fair Value
Less than 12 Months	21.6	3.1
12 Months or Greater	5.2	43.0
Total	26.8	46.1
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	0.0	(0.3)
Total	0.0	(0.3)
Corporate debt securities
Fair Value
Less than 12 Months	142.6	72.6
12 Months or Greater	2.1	330.7
Total	144.7	403.3
Unrealized Loss
Less than 12 Months	(0.1)	(0.1)
12 Months or Greater	0.0	(3.0)
Total	(0.1)	(3.1)
Foreign government debt securities
Fair Value
Less than 12 Months	4.0	1.5
12 Months or Greater	4.0	18.4
Total	8.0	19.9
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	0.0	(0.1)
Total	0.0	(0.1)
U.S. government agency securities
Fair Value
Less than 12 Months	20.0	2.0
12 Months or Greater	0.0	45.2
Total	20.0	47.2
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	0.0	(0.2)
Total	0.0	(0.2)
U.S. government securities
Fair Value
Less than 12 Months	71.6	344.0
12 Months or Greater	0.0	63.5
Total	71.6	407.5
Unrealized Loss
Less than 12 Months	(0.1)	0.0
12 Months or Greater	0.0	(0.5)
Total	$ (0.1)	$ (0.5)