Cash, Cash Equivalents and Short-Term Investments - Unrealized Loss Position Aggregated By Investment (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Less than 12 months
Fair Value	$ 78.5	$ 314.3
Gross Unrealized Losses	(0.9)	(6.0)
More than 12 months
Fair Value	575.9	633.7
Gross Unrealized Losses	(31.2)	(42.2)
Total
Fair Value	654.4	948.0
Gross Unrealized Losses	(32.1)	(48.2)
Corporate notes and obligations
Less than 12 months
Fair Value	36.5	176.5
Gross Unrealized Losses	(0.3)	(3.5)
More than 12 months
Fair Value	255.4	292.3
Gross Unrealized Losses	(15.1)	(19.2)
Total
Fair Value	291.9	468.8
Gross Unrealized Losses	(15.4)	(22.7)
U.S. Treasury securities
Less than 12 months
Fair Value	35.4	110.9
Gross Unrealized Losses	(0.6)	(1.8)
More than 12 months
Fair Value	187.2	176.9
Gross Unrealized Losses	(9.0)	(12.2)
Total
Fair Value	222.6	287.8
Gross Unrealized Losses	(9.6)	(14.0)
Asset backed securities
Less than 12 months
Fair Value	4.3	13.7
Gross Unrealized Losses	0.0	(0.4)
More than 12 months
Fair Value	71.6	90.4
Gross Unrealized Losses	(3.4)	(5.6)
Total
Fair Value	75.9	104.1
Gross Unrealized Losses	(3.4)	(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	51.1	51.6
Gross Unrealized Losses	(2.9)	(3.8)
Total
Fair Value	51.1	60.9
Gross Unrealized Losses	(2.9)	(4.1)
U.S. agency obligations
Less than 12 months
Fair Value	2.3	3.9
Gross Unrealized Losses	0.0	0.0
More than 12 months
Fair Value	3.4	13.8
Gross Unrealized Losses	(0.4)	(0.8)
Total
Fair Value	5.7	17.7
Gross Unrealized Losses	(0.4)	(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	5.6	5.5
Gross Unrealized Losses	(0.2)	(0.4)
Total
Fair Value	5.6	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	(0.2)	(0.2)
Total
Fair Value	1.6	3.2
Gross Unrealized Losses	$ (0.2)	$ (0.2)

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

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