Financial Instruments - Schedule Of Available-For-Sale Investments With Unrealized Losses (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months Fair Value	$ 519.4	$ 1,434.1
Unrealized losses less than 12 months	(0.5)	(2.8)
Unrealized losses 12 months or greater Fair Value	133.0	1,427.6
Unrealized losses 12 months or greater	(0.3)	(11.1)
Total Fair Value	652.4	2,861.7
Total Unrealized Losses	(0.8)	(13.9)
Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months Fair Value	237.0	727.4
Unrealized losses less than 12 months	(0.2)	(1.7)
Unrealized losses 12 months or greater Fair Value	0.0	409.6
Unrealized losses 12 months or greater	0.0	(3.1)
Total Fair Value	237.0	1,137.0
Total Unrealized Losses	(0.2)	(4.8)
U.S. treasuries
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months Fair Value	236.5	478.7
Unrealized losses less than 12 months	(0.2)	(0.9)
Unrealized losses 12 months or greater Fair Value	87.5	592.8
Unrealized losses 12 months or greater	(0.2)	(4.4)
Total Fair Value	324.0	1,071.5
Total Unrealized Losses	(0.4)	(5.3)
U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months Fair Value	45.9	228.0
Unrealized losses less than 12 months	(0.1)	(0.2)
Unrealized losses 12 months or greater Fair Value	45.5	425.2
Unrealized losses 12 months or greater	(0.1)	(3.6)
Total Fair Value	91.4	653.2
Total Unrealized Losses	$ (0.2)	$ (3.8)