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SPONSORED RESEARCH AND LICENSE AGREEMENTS AND GOVERNMENT CONTRACT (Details) - USD ($)	1 Months Ended							3 Months Ended			12 Months Ended	15 Months Ended
	Feb. 28, 2021	Jan. 31, 2021	Feb. 29, 2020	Dec. 31, 2019	Oct. 31, 2019	Jan. 31, 2019	Oct. 31, 2018	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Mar. 31, 2022	Dec. 31, 2021
Collaborations
Upfront payment received								$ 125,000,000.0
Accretion expense								700,000	$ 100,000
Payment of cost share to collaboration partner								2,118,000
Accounts receivable								15,240,000				$ 15,240,000	$ 15,472,000
License agreement with unrelated third party
Collaborations
Revenue recognized									4,000,000
Proceeds from License Fees Received	$ 4,000,000.0
One-time fee received from license rights granted	$ 4,000,000.0
Specified Development Events [Member]
Collaborations
Contingent payments								279,500,000
Specified Regulatory Events [Member]
Collaborations
Contingent payments								285,600,000
Specified Product Launch Events [Member]
Collaborations
Contingent payments								778,500,000
Development and regulatory milestones by non-CNS disease products
Collaborations
Contingent payments								330,000,000.0
Development and regulatory milestones by non-CNS disease products \| Milestone payments on a product-by-product basis
Collaborations
Contingent payments								100,000,000.0
Development and regulatory milestones by CNS disease products
Collaborations
Contingent payments								255,000,000.0
Development and regulatory milestones by CNS disease products \| Milestone payments on a product-by-product basis
Collaborations
Contingent payments								150,000,000.0
Maximum
Collaborations
Contingent payments								1,300,000,000
Grifols
Collaborations
Contingent payments								277,500,000
Upfront payment received						$ 30,000,000.0
Revenue, remaining performance obligation													5,000,000.0
Deferred revenue								400,000				400,000	700,000
Revenue, cumulative catch-up										$ 25,000,000.0
Grifols \| Commercial Milestones
Collaborations
Contingent payments						$ 297,500,000
Revenue recognized								1,000,000.0	1,000,000.0
Grifols \| Upon EMA approval of fostamatinib for treatment of chronic ITP
Collaborations
Revenue, cumulative catch-up										$ 20,000,000.0
Kissei
Collaborations
Contingent payments							$ 147,000,000.0
Upfront payment received							$ 33,000,000.0
Revenue, remaining performance obligation								33,000,000.0				33,000,000.0
Deferred revenue								1,400,000				1,400,000	1,400,000
fostamatinib
Collaborations
Revenue recognized								0	3,000,000.0			10,500,000
Government contract		$ 16,500,000
Remaining amount of government award expected to be received in succeeding periods								6,000,000.0
fostamatinib \| Grifols
Collaborations
Collaborative payment received			$ 20,000,000.0
Markup percentage				30.00%
fostamatinib \| Grifols \| Upon EMA approval of fostamatinib for treatment of chronic ITP
Collaborations
Contingent payments			17,500,000
fostamatinib \| Grifols \| Creditable advance royalty payment
Collaborations
Collaborative payment received			$ 2,500,000
fostamatinib \| Grifols \| Maximum
Collaborations
Royalty payment as a percentage of net sales											30.00%
fostamatinib \| Medison Pharma \| Financing arrangement
Collaborations
Upfront payment received					$ 5,000,000.0
Financing liability with accreted interest expense								$ 5,600,000				5,600,000	5,600,000
fostamatinib \| Medison Pharma \| Commercial and license agreements
Collaborations
Upfront payment received					$ 5,000,000.0
R552
Collaborations
Company's percentage of development costs								20.00%
Financing component liability								$ 57,900,000				57,900,000
Financing liability interest accretion discount rate								6.40%
Financing liability with accreted interest expense								$ 59,200,000				59,200,000	$ 60,700,000
Development costs due to collaboration partner								4,900,000				4,900,000
Payment of cost share to collaboration partner								2,100,000
Revenue, remaining performance obligation								67,100,000				67,100,000
R552 \| Maximum
Collaborations
Funding commitment								65,000,000.0				65,000,000.0
Non-CNS penetrant IP
Collaborations
Revenue recognized									60,400,000
Non-CNS penetrant IP \| Licensed Rights
Collaborations
Revenue, remaining performance obligation									6,700,000
CNS penetrant IP
Collaborations
Revenue recognized								200,000	200,000
Deferred revenue								300,000				$ 300,000
Research and development services \| Grifols
Collaborations
Revenue recognized								$ 300,000	$ 0