Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 1,455.2
Unrealized Loss - Less than 12 months	(6.2)
Fair Value - 12 Months or Greater	0.0
Unrealized Loss - 12 Months or Greater	0.0
Fair Value - Total	1,455.2
Unrealized Loss - Total	(6.2)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	1,304.1	$ 364.5
Unrealized Loss - Less than 12 months	(5.9)	(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	1,304.1	364.5
Unrealized Loss - Total	(5.9)	$ (0.2)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	123.1
Unrealized Loss - Less than 12 months	(0.2)
Fair Value - 12 Months or Greater	0.0
Unrealized Loss - 12 Months or Greater	0.0
Fair Value - Total	123.1
Unrealized Loss - Total	(0.2)
Sovereign bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	28.0
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	28.0
Unrealized Loss - Total	$ (0.1)