Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 2,039.6	$ 274.5
Unrealized Loss - Less than 12 months	(31.9)	(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	2,039.6	274.5
Unrealized Loss - Total	(31.9)	(0.3)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	1,990.1	257.2
Unrealized Loss - Less than 12 months	(31.6)	(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	1,990.1	257.2
Unrealized Loss - Total	(31.6)	(0.2)
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	49.5
Unrealized Loss - Less than 12 months	(0.3)
Fair Value - 12 Months or Greater	0.0
Unrealized Loss - 12 Months or Greater	0.0
Fair Value - Total	49.5
Unrealized Loss - Total	$ (0.3)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months		17.3
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		17.3
Unrealized Loss - Total		$ (0.1)