Fair Value Measurements - Schedule of Assets and Liabilities are Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2026	Mar. 31, 2026
Assets:
Debt securities, available-for-sale	$ 2,565,320	$ 2,872,601
Investments	504,914	407,985
Total assets at fair value	4,545,693	4,655,292
Investment in Datavant Class A units \| Common Shares
Assets:
Investments	319,223	233,171
Investment in Arbutus common shares
Assets:
Investments	185,700	174,800
Investment in Arbutus common shares \| Common Shares
Assets:
Investments	185,691	174,814
U.S. Treasury securities
Assets:
Debt securities, available-for-sale	1,879,322	2,124,179
Corporate bonds
Assets:
Debt securities, available-for-sale	668,432	748,422
Commercial paper
Assets:
Debt securities, available-for-sale	17,566	0
Level 1
Assets:
Total assets at fair value	1,661,150	1,549,520
Level 1 \| Investment in Datavant Class A units \| Common Shares
Assets:
Investments	0	0
Level 1 \| Investment in Arbutus common shares \| Common Shares
Assets:
Investments	185,691	174,814
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 \| Commercial paper
Assets:
Debt securities, available-for-sale	0	0
Level 2
Assets:
Total assets at fair value	2,565,320	2,872,601
Level 2 \| Investment in Datavant Class A units \| Common Shares
Assets:
Investments	0	0
Level 2 \| Investment in Arbutus common shares \| Common Shares
Assets:
Investments	0	0
Level 2 \| U.S. Treasury securities
Assets:
Debt securities, available-for-sale	1,879,322	2,124,179
Level 2 \| Corporate bonds
Assets:
Debt securities, available-for-sale	668,432	748,422
Level 2 \| Commercial paper
Assets:
Debt securities, available-for-sale	17,566	0
Level 3
Assets:
Total assets at fair value	319,223	233,171
Level 3 \| Investment in Datavant Class A units \| Common Shares
Assets:
Investments	319,223	233,171
Level 3 \| Investment in Arbutus common shares \| Common Shares
Assets:
Investments	0	0
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 \| Commercial paper
Assets:
Debt securities, available-for-sale	0	0
Money market funds
Assets:
Money market funds	1,475,459	1,374,706
Money market funds \| Level 1
Assets:
Money market funds	1,475,459	1,374,706
Money market funds \| Level 2
Assets:
Money market funds	0	0
Money market funds \| Level 3
Assets:
Money market funds	$ 0	$ 0