Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 1,482.6	$ 1,041.5
Unrealized Loss - Less than 12 months	(6.1)	(5.3)
Fair Value - 12 Months or Greater	900.7	324.7
Unrealized Loss - 12 Months or Greater	(12.3)	(3.6)
Fair Value - Total	2,383.3	1,366.2
Unrealized Loss - Total	(18.4)	(8.9)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	1,482.6	931.0
Unrealized Loss - Less than 12 months	(6.1)	(4.9)
Fair Value - 12 Months or Greater	801.6	256.8
Unrealized Loss - 12 Months or Greater	(11.3)	(2.8)
Fair Value - Total	2,284.2	1,187.8
Unrealized Loss - Total	(17.4)	(7.7)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	0.0	110.5
Unrealized Loss - Less than 12 months	0.0	(0.4)
Fair Value - 12 Months or Greater	99.1	67.9
Unrealized Loss - 12 Months or Greater	(1.0)	(0.8)
Fair Value - Total	99.1	178.4
Unrealized Loss - Total	$ (1.0)	$ (1.2)