Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 1,311.7
Unrealized Loss - Less than 12 months	(3.7)
Fair Value - 12 Months or Greater	0.0
Unrealized Loss - 12 Months or Greater	0.0
Fair Value - Total	1,311.7
Unrealized Loss - Total	(3.7)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	1,167.0	$ 364.5
Unrealized Loss - Less than 12 months	(3.5)	(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	1,167.0	364.5
Unrealized Loss - Total	(3.5)	$ (0.2)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	111.3
Unrealized Loss - Less than 12 months	(0.1)
Fair Value - 12 Months or Greater	0.0
Unrealized Loss - 12 Months or Greater	0.0
Fair Value - Total	111.3
Unrealized Loss - Total	(0.1)
Sovereign bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	33.4
Unrealized Loss - Less than 12 months	(0.1)
Fair Value - 12 Months or Greater	0.0
Unrealized Loss - 12 Months or Greater	0.0
Fair Value - Total	33.4
Unrealized Loss - Total	$ (0.1)