Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 517.3	$ 1,482.6
Unrealized Loss - Less than 12 months	(0.3)	(6.1)
Fair Value - 12 Months or Greater	1,033.4	900.7
Unrealized Loss - 12 Months or Greater	(6.6)	(12.3)
Fair Value - Total	1,550.7	2,383.3
Unrealized Loss - Total	(6.9)	(18.4)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	313.3	1,482.6
Unrealized Loss - Less than 12 months	(0.3)	(6.1)
Fair Value - 12 Months or Greater	941.0	801.6
Unrealized Loss - 12 Months or Greater	(6.0)	(11.3)
Fair Value - Total	1,254.3	2,284.2
Unrealized Loss - Total	(6.3)	(17.4)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	204.0	0.0
Unrealized Loss - Less than 12 months	0.0	0.0
Fair Value - 12 Months or Greater	92.4	99.1
Unrealized Loss - 12 Months or Greater	(0.6)	(1.0)
Fair Value - Total	296.4	99.1
Unrealized Loss - Total	$ (0.6)	$ (1.0)