Cash, Cash Equivalents and Short-Term Investments - Unrealized Loss Position Aggregated By Investment (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Less than 12 months
Fair Value	$ 43.5	$ 314.3
Gross Unrealized Losses	(0.2)	(6.0)
More than 12 months
Fair Value	534.8	633.7
Gross Unrealized Losses	(21.1)	(42.2)
Total
Fair Value	578.3	948.0
Gross Unrealized Losses	(21.3)	(48.2)
Corporate notes and obligations
Less than 12 months
Fair Value	25.1	176.5
Gross Unrealized Losses	(0.1)	(3.5)
More than 12 months
Fair Value	240.3	292.3
Gross Unrealized Losses	(10.2)	(19.2)
Total
Fair Value	265.4	468.8
Gross Unrealized Losses	(10.3)	(22.7)
U.S. Treasury securities
Less than 12 months
Fair Value	17.8	110.9
Gross Unrealized Losses	(0.1)	(1.8)
More than 12 months
Fair Value	174.0	176.9
Gross Unrealized Losses	(6.0)	(12.2)
Total
Fair Value	191.8	287.8
Gross Unrealized Losses	(6.1)	(14.0)
Asset backed securities
Less than 12 months
Fair Value	0.6	13.7
Gross Unrealized Losses	0.0	(0.4)
More than 12 months
Fair Value	66.0	90.4
Gross Unrealized Losses	(2.3)	(5.6)
Total
Fair Value	66.6	104.1
Gross Unrealized Losses	(2.3)	(6.0)
Municipal securities
Less than 12 months
Fair Value	0.0	9.3
Gross Unrealized Losses	0.0	(0.3)
More than 12 months
Fair Value	46.1	51.6
Gross Unrealized Losses	(2.0)	(3.8)
Total
Fair Value	46.1	60.9
Gross Unrealized Losses	(2.0)	(4.1)
U.S. agency obligations
Less than 12 months
Fair Value	0.0	3.9
Gross Unrealized Losses	0.0	0.0
More than 12 months
Fair Value	3.5	13.8
Gross Unrealized Losses	(0.3)	(0.8)
Total
Fair Value	3.5	17.7
Gross Unrealized Losses	(0.3)	(0.8)
Foreign government obligations
Less than 12 months
Fair Value	0.0	0.0
Gross Unrealized Losses	0.0	0.0
More than 12 months
Fair Value	3.3	5.5
Gross Unrealized Losses	(0.2)	(0.4)
Total
Fair Value	3.3	5.5
Gross Unrealized Losses	(0.2)	(0.4)
Supranational securities
Less than 12 months
Fair Value	0.0	0.0
Gross Unrealized Losses	0.0	0.0
More than 12 months
Fair Value	1.6	3.2
Gross Unrealized Losses	(0.1)	(0.2)
Total
Fair Value	1.6	3.2
Gross Unrealized Losses	$ (0.1)	$ (0.2)

Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions

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