Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 313.3	$ 1,482.6
Unrealized Loss - Less than 12 months	(0.4)	(6.1)
Fair Value - 12 Months or Greater	235.8	900.7
Unrealized Loss - 12 Months or Greater	(0.3)	(12.3)
Fair Value - Total	549.1	2,383.3
Unrealized Loss - Total	(0.7)	(18.4)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	294.8	1,482.6
Unrealized Loss - Less than 12 months	(0.3)	(6.1)
Fair Value - 12 Months or Greater	216.3	801.6
Unrealized Loss - 12 Months or Greater	(0.3)	(11.3)
Fair Value - Total	511.1	2,284.2
Unrealized Loss - Total	(0.6)	(17.4)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	18.5	0.0
Unrealized Loss - Less than 12 months	(0.1)	0.0
Fair Value - 12 Months or Greater	19.5	99.1
Unrealized Loss - 12 Months or Greater	0.0	(1.0)
Fair Value - Total	38.0	99.1
Unrealized Loss - Total	$ (0.1)	$ (1.0)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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