Fair Value Measurements - Schedule of Assets and Liabilities are Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Mar. 31, 2026
Assets:
Debt securities, available-for-sale	$ 2,593,626	$ 2,872,601
Investment	444,622	407,985
Total assets at fair value	4,209,616	4,655,292
U.S. Treasury securities
Assets:
Debt securities, available-for-sale	1,888,731	2,124,179
Corporate bonds
Assets:
Debt securities, available-for-sale	704,895	748,422
Money market funds
Assets:
Money market funds	1,171,368	1,374,706
Level 1
Assets:
Total assets at fair value	1,357,836	1,549,520
Level 1 \| U.S. Treasury securities
Assets:
Debt securities, available-for-sale	0	0
Level 1 \| Corporate bonds
Assets:
Debt securities, available-for-sale	0	0
Level 1 \| Money market funds
Assets:
Money market funds	1,171,368	1,374,706
Level 2
Assets:
Total assets at fair value	2,593,626	2,872,601
Level 2 \| U.S. Treasury securities
Assets:
Debt securities, available-for-sale	1,888,731	2,124,179
Level 2 \| Corporate bonds
Assets:
Debt securities, available-for-sale	704,895	748,422
Level 2 \| Money market funds
Assets:
Money market funds	0	0
Level 3
Assets:
Total assets at fair value	258,154	233,171
Level 3 \| U.S. Treasury securities
Assets:
Debt securities, available-for-sale	0	0
Level 3 \| Corporate bonds
Assets:
Debt securities, available-for-sale	0	0
Level 3 \| Money market funds
Assets:
Money market funds	0	0
Investment in Arbutus common shares
Assets:
Investment	186,500	174,800
Common Shares \| Investment in Datavant Class A units
Assets:
Investment	258,154	233,171
Common Shares \| Investment in Datavant Class A units \| Level 1
Assets:
Investment	0	0
Common Shares \| Investment in Datavant Class A units \| Level 2
Assets:
Investment	0	0
Common Shares \| Investment in Datavant Class A units \| Level 3
Assets:
Investment	258,154	233,171
Common Shares \| Investment in Arbutus common shares
Assets:
Investment	186,468	174,814
Common Shares \| Investment in Arbutus common shares \| Level 1
Assets:
Investment	186,468	174,814
Common Shares \| Investment in Arbutus common shares \| Level 2
Assets:
Investment	0	0
Common Shares \| Investment in Arbutus common shares \| Level 3
Assets:
Investment	$ 0	$ 0