Investments - Gross Unrealized Losses and Fair Values of Investments in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Mar. 31, 2023
Fair Value
Less than 12 months	$ 210,249	$ 142,875
12 months or greater	377,703	510,497
Total, fair value	587,952	653,372
Gross Unrealized Losses
Less than 12 months	(281)	(331)
12 months or greater	(11,706)	(18,593)
Total, unrealized losses	(11,987)	(18,924)
Asset-backed securities
Fair Value
Less than 12 months	4,569	2,601
12 months or greater	1,018	4,599
Total, fair value	5,587	7,200
Gross Unrealized Losses
Less than 12 months	(8)	(12)
12 months or greater	(4)	(59)
Total, unrealized losses	(12)	(71)
Certificates of deposit
Fair Value
Less than 12 months	11,006	27,018
12 months or greater	0	0
Total, fair value	11,006	27,018
Gross Unrealized Losses
Less than 12 months	(15)	(80)
12 months or greater	0	0
Total, unrealized losses	(15)	(80)
Commercial paper
Fair Value
Less than 12 months	96,131	70,681
12 months or greater	0	0
Total, fair value	96,131	70,681
Gross Unrealized Losses
Less than 12 months	(44)	(126)
12 months or greater	0	0
Total, unrealized losses	(44)	(126)
Corporate notes and bonds
Fair Value
Less than 12 months	55,618	42,575
12 months or greater	6,241	58,766
Total, fair value	61,859	101,341
Gross Unrealized Losses
Less than 12 months	(139)	(113)
12 months or greater	(127)	(665)
Total, unrealized losses	(266)	(778)
Sovereign bonds
Fair Value
Less than 12 months	0	0
12 months or greater	7,477	7,384
Total, fair value	7,477	7,384
Gross Unrealized Losses
Less than 12 months	0	0
12 months or greater	(269)	(360)
Total, unrealized losses	(269)	(360)
U.S. government and agency securities
Fair Value
Less than 12 months	42,925	0
12 months or greater	362,967	439,748
Total, fair value	405,892	439,748
Gross Unrealized Losses
Less than 12 months	(75)	0
12 months or greater	(11,306)	(17,509)
Total, unrealized losses	$ (11,381)	$ (17,509)

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