Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Schedule of Fair Value Hierarchy for Financial Assets

The following table summarizes financial assets that we measured at fair value on a recurring basis as of December 31, 2023, classified in accordance with the fair value hierarchy:

	Fair Value Measurements Using
(In millions)	Level 1		Level 2		Level 3		Total
Cash equivalents	$	315.9	$	—	$	—	$	315.9
Debt securities, available-for-sale:
U.S. government agencies (1)	—		1,612.5		—		1,612.5
Commercial paper	—		184.7		—		184.7
Corporate debt	—		360.6		—		360.6
Total debt securities, available-for-sale	—		2,157.8		—		2,157.8
Other assets (2)	15.2		—		—		15.2
Total assets measured at fair value on a recurring basis	$	331.1	$	2,157.8	$	—	$	2,488.9

The following table summarizes financial assets that we measured at fair value on a recurring basis as of December 31, 2022, classified in accordance with the fair value hierarchy:

	Fair Value Measurements Using
(In millions)	Level 1		Level 2		Level 3		Total
Cash equivalents	$	375.9	$	44.8	$	—	$	420.7
Debt securities, available-for-sale:
U.S. government agencies (1)	—		1,530.7		—		1,530.7
Commercial paper	—		119.4		—		119.4
Corporate debt	—		163.8		—		163.8
Total debt securities, available-for-sale	—		1,813.9		—		1,813.9
Other assets (2)	10.2		—		—		10.2
Total assets measured at fair value on a recurring basis	$	386.1	$	1,858.7	$	—	$	2,244.8

(1) Includes debt obligations issued by U.S. government-sponsored enterprises or U.S. government agencies.

(2) Includes assets which are held pursuant to a deferred compensation plan for senior management, which consist mainly of mutual funds.

Schedule of Fair Value of Senior Convertible Notes

The fair value, based on trading prices (Level 1 inputs), of our senior convertible notes were as follows as of the dates indicated:

	Fair Value Measurements Using Level 1
(In millions)	December 31, 2023		December 31, 2022
Senior Convertible Notes due 2023	$	—	$	2,136.2
Senior Convertible Notes due 2025	1,262.8		1,314.9
Senior Convertible Notes due 2028	1,281.8		—
Total fair value of outstanding senior convertible notes	$	2,544.6	$	3,451.1