Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 274.5	$ 1,482.6
Unrealized Loss - Less than 12 months	(0.3)	(6.1)
Fair Value - 12 Months or Greater	43.1	900.7
Unrealized Loss - 12 Months or Greater	0.0	(12.3)
Fair Value - Total	317.6	2,383.3
Unrealized Loss - Total	(0.3)	(18.4)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	257.2	1,482.6
Unrealized Loss - Less than 12 months	(0.2)	(6.1)
Fair Value - 12 Months or Greater	41.1	801.6
Unrealized Loss - 12 Months or Greater	0.0	(11.3)
Fair Value - Total	298.3	2,284.2
Unrealized Loss - Total	(0.2)	(17.4)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	17.3	0.0
Unrealized Loss - Less than 12 months	(0.1)	0.0
Fair Value - 12 Months or Greater	2.0	99.1
Unrealized Loss - 12 Months or Greater	0.0	(1.0)
Fair Value - Total	19.3	99.1
Unrealized Loss - Total	$ (0.1)	$ (1.0)