Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 606.0	$ 274.5
Unrealized Loss - Less than 12 months	(0.8)	(0.3)
Fair Value - 12 Months or Greater	0.0	0.0
Unrealized Loss - 12 Months or Greater	0.0	0.0
Fair Value - Total	606.0	274.5
Unrealized Loss - Total	(0.8)	(0.3)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	560.5	257.2
Unrealized Loss - Less than 12 months	(0.7)	(0.2)
Fair Value - 12 Months or Greater	0.0	0.0
Unrealized Loss - 12 Months or Greater	0.0	0.0
Fair Value - Total	560.5	257.2
Unrealized Loss - Total	(0.7)	(0.2)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	45.5	17.3
Unrealized Loss - Less than 12 months	(0.1)	(0.1)
Fair Value - 12 Months or Greater	0.0	0.0
Unrealized Loss - 12 Months or Greater	0.0	0.0
Fair Value - Total	45.5	17.3
Unrealized Loss - Total	$ (0.1)	$ (0.1)