Investments - Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Marketable investments classified as available-for-sale securities:
Amortized Cost	$ 1,511.5	$ 1,149.7
Gross Unrealized Gains	5.0	0.7
Gross Unrealized Losses	(0.6)	(4.2)
Fair Value	1,515.9	1,146.2
U.S. government and agency securities
Marketable investments classified as available-for-sale securities:
Amortized Cost	1,287.9	1,077.4
Gross Unrealized Gains	3.2	0.7
Gross Unrealized Losses	(0.5)	(3.9)
Fair Value	1,290.6	1,074.2
Corporate debt securities
Marketable investments classified as available-for-sale securities:
Amortized Cost	223.6	72.3
Gross Unrealized Gains	1.8	0.0
Gross Unrealized Losses	(0.1)	(0.3)
Fair Value	$ 225.3	$ 72.0