Cash Equivalents and Investments - Unrealized Loss Position on Fixed Income Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 Months	$ 365.0	$ 423.2
12 Months or Greater	393.1	500.8
Total	758.1	924.0
Unrealized Loss
Less than 12 Months	0.0	(0.1)
12 Months or Greater	(1.9)	(4.1)
Total	(1.9)	(4.2)
Asset-backed securities
Fair Value
Less than 12 Months	0.2	3.1
12 Months or Greater	27.7	43.0
Total	27.9	46.1
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	(0.1)	(0.3)
Total	(0.1)	(0.3)
Corporate debt securities
Fair Value
Less than 12 Months	47.3	72.6
12 Months or Greater	278.1	330.7
Total	325.4	403.3
Unrealized Loss
Less than 12 Months	0.0	(0.1)
12 Months or Greater	(1.3)	(3.0)
Total	(1.3)	(3.1)
Foreign government debt securities
Fair Value
Less than 12 Months	5.5	1.5
12 Months or Greater	17.3	18.4
Total	22.8	19.9
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	(0.1)	(0.1)
Total	(0.1)	(0.1)
U.S. government agency securities
Fair Value
Less than 12 Months	0.0	2.0
12 Months or Greater	23.6	45.2
Total	23.6	47.2
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	(0.1)	(0.2)
Total	(0.1)	(0.2)
U.S. government securities
Fair Value
Less than 12 Months	312.0	344.0
12 Months or Greater	46.4	63.5
Total	358.4	407.5
Unrealized Loss
Less than 12 Months	0.0	0.0
12 Months or Greater	(0.3)	(0.5)
Total	$ (0.3)	$ (0.5)