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Collaboration and Licensing Agreements (Details)	1 Months Ended										3 Months Ended			12 Months Ended
Collaboration and Licensing Agreements (Details)	Oct. 31, 2017 USD ($) shares	Mar. 31, 2017 USD ($)	Jul. 31, 2016 USD ($)	Jun. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Jul. 31, 2013 USD ($) item	Jan. 31, 2013 USD ($) item	Jun. 30, 2010 USD ($)	Feb. 28, 2009 item	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2007 item
License Agreement \| Merck Sharp & Dohme Corp.
Collaboration research and licensing agreements
Nonrefundable upfront payment							$ 1,000,000
Revenue recognized														$ 0	$ 0	$ 0
Deferred revenue											$ 0			0
Annual maintenance fees							$ 500,000
Number of compounds \| item							1
License Agreement \| INmune
Collaboration research and licensing agreements
Nonrefundable upfront payment	$ 100,000
Fully-diluted equity interests (as a percentage)	19.00%
Additional equity interests (as a percentage)	10.00%
Equity issued in shares \| shares	1,585,000
Equity issues, value	$ 10,000,000
Revenue recognized														0	100,000
Deferred revenue											0			0
Option and license agreement \| Alexion Pharmaceuticals, Inc.
Collaboration research and licensing agreements
Nonrefundable upfront payment								$ 3,000,000
Revenue recognized											11,000,000	$ 9,000,000		20,000,000	0	5,000,000
Deferred revenue											0			0
Number of different target programs \| item								6
Research license term								5 years
Annual maintenance fees								$ 500,000
Milestone payment received														36,500,000
2009 Research License and Commercialization Agreement \| CSL Limited
Collaboration research and licensing agreements
Revenue recognized		$ 3,500,000												0	3,500,000	0
Deferred revenue											0			0
Collaboration And License Agreement \| Novartis
Collaboration research and licensing agreements
Nonrefundable upfront payment			$ 150,000,000
Revenue recognized														20,000,000	20,100,000	69,900,000
Deferred revenue											40,100,000		$ 60,100,000	40,100,000	60,100,000
Contract receivable											2,100,000			2,100,000
Collaboration And License Agreement \| Novartis \| Maximum
Collaboration research and licensing agreements
Potential milestone payment											2,100,000,000			$ 2,100,000,000
Collaboration And License Agreement \| Novartis \| Discovery Program
Collaboration research and licensing agreements
Right to substitute identified target period														5 years
Collaboration And License Agreement \| Novartis \| Bispecific FC Technologies
Collaboration research and licensing agreements
Number of development stage products \| item				2
Number of previously identified products \| item				4
Research license term				5 years
Collaboration And License Agreement \| Novartis \| Bispecific FC Technologies \| XmAb14045
Collaboration research and licensing agreements
Allocation of consideration to deliverables				$ 27,100,000
Collaboration And License Agreement \| Novartis \| Bispecific FC Technologies \| XmAb13676
Collaboration research and licensing agreements
Allocation of consideration to deliverables				$ 31,400,000
Collaboration And License Agreement \| Novartis \| Bispecific FC Technologies \| Maximum
Collaboration research and licensing agreements
Number of antibody targets for which bispecific technology applied \| item				4
Collaboration And License Agreement \| Novartis \| Global Discovery Program
Collaboration research and licensing agreements
Number of programs where Company has the right to participate \| item				1
Revenue recognized												$ 20,040,000			20,050,000
Percentage of responsibility for development costs				25.00%
Percentage of responsibility for commercialization costs				50.00%
Percentage of profits on net sales of the product				50.00%
Allocation of consideration to deliverables				$ 20,050,000
Number of discovery programs \| item				4
Collaboration And License Agreement \| Novartis \| Global Discovery Program \| Maximum
Collaboration research and licensing agreements
Number of antibody targets for which bispecific technology applied \| item				4
Collaboration And License Agreement \| Novartis \| FC Licenses
Collaboration research and licensing agreements
Allocation of consideration to deliverables				$ 11,300,000
Collaboration And License Agreement \| Novartis \| FC Licenses \| Maximum
Collaboration research and licensing agreements
Number of targets against which non-exclusive license is provided \| item				10
Collaboration And License Agreement \| Novo Nordisk A/S
Collaboration research and licensing agreements
Nonrefundable upfront payment						$ 2,500,000
Revenue recognized														$ 0	0	2,700,000
Deferred revenue											0			0
Research license term						2 years
Milestone payment received						$ 1,600,000
Collaboration And License Agreement \| MorphoSys
Collaboration research and licensing agreements
Nonrefundable upfront payment									$ 13,000,000
Revenue recognized														0	12,500,000	0
Deferred revenue											0			0
Research and License Agreement 2015 \| Amgen, Inc.
Collaboration research and licensing agreements
Nonrefundable upfront payment					$ 45,000,000
Potential milestone payment					$ 600,000,000
Revenue recognized														600,000	10,000,000	31,200,000
Deferred revenue											0			0
Allocation of consideration to deliverables											55,500,000			55,500,000
Research and License Agreement 2015 \| Amgen, Inc. \| ASU 2014-09
Collaboration research and licensing agreements
Revenue recognized																31,250,000
Research and License Agreement 2015 \| Amgen, Inc. \| CD38 Program
Collaboration research and licensing agreements
Revenue recognized													$ 10,000,000
Allocation of consideration to deliverables											23,750,000			23,750,000
Research and License Agreement 2015 \| Amgen, Inc. \| Discovery Program
Collaboration research and licensing agreements
Revenue recognized											500,000
Research license term					3 years
Allocation of consideration to deliverables											6,250,000			6,250,000
Number of discovery programs \| item					5
Additional research term					1 year
Previous targets which bispecific technology will be applied \| item					5
Number of delivered discover programs \| item					5
Research Licensee and Collaboration Agreement 2007 Agreement \| CSL Limited
Collaboration research and licensing agreements
Number of commercial licenses \| item										1
Research Licensee and Collaboration Agreement 2007 Agreement \| Boehringer Ingelheim International GmbH
Collaboration research and licensing agreements
Revenue recognized														0	$ 0	$ 0
Deferred revenue											$ 0			$ 0
Research license term																	3 years
Number of commercial options \| item																	1
Number of compounds \| item															1
Number of commercial licenses \| item															2