Investments - Available-for-Sale (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Marketable investments classified as available-for-sale securities:
Amortized Cost	$ 1,223.0	$ 1,149.7
Gross Unrealized Gains	1.8	0.7
Gross Unrealized Losses	(2.0)	(4.2)
Fair Value	1,222.8	1,146.2
U.S. government and agency securities
Marketable investments classified as available-for-sale securities:
Amortized Cost	1,111.8	1,077.4
Gross Unrealized Gains	1.3	0.7
Gross Unrealized Losses	(2.0)	(3.9)
Fair Value	1,111.1	1,074.2
Corporate debt securities
Marketable investments classified as available-for-sale securities:
Amortized Cost	111.2	72.3
Gross Unrealized Gains	0.5
Gross Unrealized Losses		(0.3)
Fair Value	$ 111.7	$ 72.0