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Collaboration and Licensing Agreements (Details)				1 Months Ended										3 Months Ended				12 Months Ended
Collaboration and Licensing Agreements (Details)	Feb. 04, 2020 USD ($) shares	Mar. 29, 2019 USD ($)	Mar. 08, 2019	Dec. 31, 2020 USD ($) shares	Nov. 30, 2020 USD ($) item	Aug. 31, 2020 USD ($) item	Jul. 31, 2020 USD ($) Program	Feb. 29, 2020 USD ($)	Jan. 31, 2020 USD ($) item	Jun. 30, 2019 USD ($)	Feb. 28, 2019	Oct. 31, 2017 USD ($) shares	Jan. 31, 2013 Program	Mar. 31, 2021 USD ($) item	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($) item	Dec. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	May 31, 2018 USD ($)
Collaboration research and licensing agreements
Other income														$ 12,970,000		$ (2,336,000)
Deferred revenue														77,800,000		46,200,000
Receivable				$ 11,443,000										$ 12,525,000	$ 11,443,000			$ 11,443,000
Atreca, Inc. \| Collaboration and License Agreement
Collaboration research and licensing agreements
Research license term							3 years
Sharing percent of costs and profits							50.00%
Novartis
Collaboration research and licensing agreements
Number of performance obligations														2
License, Development, and Commercialization Agreement \| Aimmune
Collaboration research and licensing agreements
Nonrefundable upfront payment	$ 5,000,000.0
Revenue recognized														$ 0		9,600,000
Shares received in noncash transaction \| shares	156,238
Purchase amount of share options or equity in noncash transaction	$ 4,600,000
Deferred revenue														0
License, Development, and Commercialization Agreement \| Aimmune \| Maximum
Collaboration research and licensing agreements
Potential milestone payment	385,000,000.0
Option and License Agreement \| Alexion
Collaboration research and licensing agreements
Deferred revenue														0
Receivable														9,500,000
Number of different target programs \| Program													1
Option and License Agreement \| Alexion \| Royalty
Collaboration research and licensing agreements
Revenue recognized														5,300,000		3,300,000
Option and License Agreement \| Alexion \| Milestone
Collaboration research and licensing agreements
Contract asset				10,000,000.0											10,000,000.0			10,000,000.0
Research and License Agreement \| Amgen, Inc.
Collaboration research and licensing agreements
Revenue recognized														0		0
Deferred revenue														0
Research and License Agreement \| Astellas
Collaboration research and licensing agreements
Nonrefundable upfront payment		$ 15,000,000.0
Potential milestone payment		240,000,000.0
Revenue recognized														0		300,000
Deferred revenue														0
Research and License Agreement \| Astellas \| Bispecific
Collaboration research and licensing agreements
Revenue recognized										$ 13,600,000
Research and License Agreement \| Astellas \| Research service
Collaboration research and licensing agreements
Performance obligation		1,400,000
Research and License Agreement \| Astellas \| Milestone
Collaboration research and licensing agreements
Contract asset				2,500,000											2,500,000			2,500,000
Research and License Agreement \| Vir
Collaboration research and licensing agreements
Potential milestone payment																	$ 155,000,000.0
Revenue recognized														0		0
Deferred revenue														0
Number of different target programs \| item																	2
Collaboration and License Agreement \| Atreca, Inc.
Collaboration research and licensing agreements
Number of programs \| Program							1
Number of different target programs \| Program							2
Collaboration and License Agreement \| Genentech
Collaboration research and licensing agreements
Initial cost-sharing percentage			45.00%
Research license term											2 years
Revenue recognized														200,000		700,000
Collaboration and License Agreement \| Genentech \| XmAb24306
Collaboration research and licensing agreements
Cost sharing receivable (payable)														(2,900,000)
Collaboration and License Agreement \| Genentech \| Research service
Collaboration research and licensing agreements
Performance obligation																			$ 8,300,000
Deferred revenue														2,300,000
Collaboration and License Agreement \| Genentech \| Licensing
Collaboration research and licensing agreements
Performance obligation																			$ 111,700,000
Collaboration and License Agreement \| Janssen Biotech, Inc
Collaboration research and licensing agreements
Performance obligation					$ 50,000,000.0
Nonrefundable upfront payment					50,000,000.0
Potential milestone payment					662,500,000
Revenue recognized														14,600,000
Deferred revenue														35,400,000
Collaboration and License Agreement \| MorphoSys
Collaboration research and licensing agreements
Revenue recognized							$ 25,000,000.0
Contract asset														12,500,000
Deferred revenue														0
Collaboration and License Agreement \| MorphoSys \| Royalty
Collaboration research and licensing agreements
Revenue recognized														1,400,000
Receivable														1,400,000
Collaboration and License Agreement \| MorphoSys \| Milestone
Collaboration research and licensing agreements
Revenue recognized								$ 12,500,000						12,500,000		12,500,000
Collaboration and License Agreement \| Novartis
Collaboration research and licensing agreements
Revenue recognized														0		0
Deferred revenue														40,100,000
Cost sharing receivable (payable)														$ 900,000
Collaboration and License Agreement \| Novartis \| Discovery Program
Collaboration research and licensing agreements
Number of performance obligations \| item														2
Technology License Agreement \| Quellis \| Maximum
Collaboration research and licensing agreements
Potential milestone payment																				$ 66,000,000.0
Technology License Agreement \| Catabasis
Collaboration research and licensing agreements
Other income														$ 12,900,000
Deferred revenue														0
Technology License Agreement \| Gilead
Collaboration research and licensing agreements
Nonrefundable upfront payment									$ 6,000,000.0
Potential milestone payment									$ 67,000,000.0
Revenue recognized														0		6,000,000.0
Deferred revenue														0
Number of compounds \| item									3
Technology License Agreement \| MiRagen/Viridian
Collaboration research and licensing agreements
Potential milestone payment				$ 55,000,000.0											55,000,000.0			55,000,000.0
Other income														100,000
Revenue recognized														0				6,000,000.0
Shares received in noncash transaction \| shares				322,407
Purchase amount of share options or equity in noncash transaction				$ 6,000,000.0
Deferred revenue														0
Technology License Agreement \| Omeros
Collaboration research and licensing agreements
Nonrefundable upfront payment						$ 5,000,000.0
Potential milestone payment						$ 65,000,000.0
Revenue recognized														0
Deferred revenue														0
Number of additional antibodies \| item						3
License Agreement \| INmune
Collaboration research and licensing agreements
Revenue recognized														0		0
Shares received in noncash transaction \| shares												1,585,000
Purchase amount of share options or equity in noncash transaction												$ 10,000,000.0
Deferred revenue														0
Equity securities														0		$ 0
Option term												6 years
License Agreement \| INmune \| Maximum
Collaboration research and licensing agreements
Additional equity interests (as a percentage)												10.00%
License Agreement \| Zenas Bio Pharma Limited
Collaboration research and licensing agreements
Performance obligation					16,100,000
Revenue recognized														0
Purchase amount of share options or equity in noncash transaction					$ 16,100,000
Deferred revenue														0
Percentage of equity of private company					15.00%
Number of drug candidates \| item					3
License Agreement \| Zenas Bio Pharma Limited \| Licensing
Collaboration research and licensing agreements
Revenue recognized															16,100,000
Development-based \| License, Development, and Commercialization Agreement \| Aimmune
Collaboration research and licensing agreements
Potential milestone payment	22,000,000.0
Development-based \| Research and License Agreement \| Astellas
Collaboration research and licensing agreements
Potential milestone payment		32,500,000
Development-based \| Research and License Agreement \| Vir
Collaboration research and licensing agreements
Potential milestone payment																	$ 5,000,000.0
Development-based \| Collaboration and License Agreement \| Janssen Biotech, Inc
Collaboration research and licensing agreements
Potential milestone payment					$ 161,900,000
Development-based \| Collaboration and License Agreement \| MorphoSys
Collaboration research and licensing agreements
Contract asset														$ 12,500,000
Development-based \| Technology License Agreement \| Quellis
Collaboration research and licensing agreements
Potential milestone payment																				6,000,000.0
Development-based \| Technology License Agreement \| Gilead
Collaboration research and licensing agreements
Potential milestone payment									$ 10,000,000.0
Development-based \| Technology License Agreement \| MiRagen/Viridian
Collaboration research and licensing agreements
Potential milestone payment				10,000,000.0											10,000,000.0			10,000,000.0
Development-based \| Technology License Agreement \| Omeros
Collaboration research and licensing agreements
Potential milestone payment						$ 15,000,000.0
Regulatory-based \| License, Development, and Commercialization Agreement \| Aimmune
Collaboration research and licensing agreements
Potential milestone payment	53,000,000.0
Regulatory-based \| Research and License Agreement \| Astellas
Collaboration research and licensing agreements
Potential milestone payment		57,500,000
Regulatory-based \| Research and License Agreement \| Vir
Collaboration research and licensing agreements
Potential milestone payment																	30,000,000.0
Regulatory-based \| Collaboration and License Agreement \| Janssen Biotech, Inc
Collaboration research and licensing agreements
Potential milestone payment					240,600,000
Regulatory-based \| Technology License Agreement \| Quellis
Collaboration research and licensing agreements
Potential milestone payment																				30,000,000.0
Regulatory-based \| Technology License Agreement \| Gilead
Collaboration research and licensing agreements
Potential milestone payment									27,000,000.0
Regulatory-based \| Technology License Agreement \| MiRagen/Viridian
Collaboration research and licensing agreements
Potential milestone payment				20,000,000.0											20,000,000.0			20,000,000.0
Regulatory-based \| Technology License Agreement \| Omeros
Collaboration research and licensing agreements
Potential milestone payment						25,000,000.0
Sales-based \| License, Development, and Commercialization Agreement \| Aimmune
Collaboration research and licensing agreements
Potential milestone payment	$ 310,000,000.0
Sales-based \| Research and License Agreement \| Astellas
Collaboration research and licensing agreements
Potential milestone payment		$ 150,000,000.0
Sales-based \| Research and License Agreement \| Vir
Collaboration research and licensing agreements
Potential milestone payment																	$ 120,000,000.0
Sales-based \| Collaboration and License Agreement \| Janssen Biotech, Inc
Collaboration research and licensing agreements
Potential milestone payment					$ 260,000,000.0
Sales-based \| Technology License Agreement \| Quellis
Collaboration research and licensing agreements
Potential milestone payment																				$ 30,000,000.0
Sales-based \| Technology License Agreement \| Gilead
Collaboration research and licensing agreements
Potential milestone payment									$ 30,000,000.0
Sales-based \| Technology License Agreement \| MiRagen/Viridian
Collaboration research and licensing agreements
Potential milestone payment				$ 25,000,000.0											$ 25,000,000.0			$ 25,000,000.0
Sales-based \| Technology License Agreement \| Omeros
Collaboration research and licensing agreements
Potential milestone payment						$ 25,000,000.0