Collaboration Agreements - Narrative (Detail)		1 Months Ended	3 Months Ended	12 Months Ended			60 Months Ended
	Jan. 02, 2019	Jun. 30, 2014 USD ($) target milestone	Mar. 31, 2019 USD ($)	Dec. 31, 2021 USD ($) target	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) agreement	Dec. 31, 2019 USD ($) agreement	Jan. 01, 2018 USD ($)	Jan. 31, 2016 target
Collaboration agreements
Deferred revenue				$ 3,944,000	$ 3,987,000	$ 4,815,000	$ 4,815,000
Amounts included in the contract liability at the beginning of the period				43,000	828,000
Collaboration revenue				43,000	828,000	42,123,000
Research and development				132,013,000	67,036,000	55,040,000
Merck KGaA
Collaboration agreements
Upfront payment received		$ 12,000,000
Number of targets | target		6
Amount of additional fees receivable when target is designated and commercial license to target is granted		$ 0
Aggregate milestones		780,000,000
Development milestones		84,000,000
Regulatory milestones		264,000,000
Sales milestones		$ 432,000,000
Number of individual development milestones per target | milestone		6
Number of individual regulatory milestones per target | milestone		5
Number of individual commercial milestones | milestone		3
Amount of development milestone payment received				0	0		$ 3,000,000
Next potential development milestone payment eligible to receive				$ 500,000
Prior written notice for termination period				60 days
Number of targets designated | target				6
Revenue, transaction price, amount				$ 21,325,000	21,325,000
Number of service obligations completed of designated targets | target				4
Collaboration revenue				$ 43,000	828,000	853,000
Milestone Payment Not Received, Included in Estimated Transaction Price								$ 0
Merck KGaA | License and corresponding research and development services, first and second targets
Collaboration agreements
Revenue, transaction price, amount				3,941,000
Merck KGaA | License and corresponding research and development services, third and sixth target
Collaboration agreements
Revenue, transaction price, amount				3,439,000
Merck KGaA | License and corresponding research and development services, fourth target
Collaboration agreements
Revenue, transaction price, amount				3,152,000
Merck KGaA | License and corresponding research and development services, fifth target
Collaboration agreements
Revenue, transaction price, amount				2,746,000
Merck KGaA | Rights to future technological improvements
Collaboration agreements
Revenue, transaction price, amount				$ 425,000
Merck KGaA | Rights to future technological improvements | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Collaboration agreements
Expected recognition period				10 years
Merck KGaA | Joint research committee services
Collaboration agreements
Revenue, transaction price, amount				$ 242,000
Merck KGaA | Joint research committee services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Collaboration agreements
Expected recognition period				5 years
Merck KGaA Agreement and Merck KGaA supply agreement
Collaboration agreements
Accounts receivable				$ 0	0
Deferred revenue				3,944,000	3,987,000
XMT-1522 agreement
Collaboration agreements
Period of time until agreement terminated	30 days
Period of shared development costs following effective termination date	30 days
Research and development						200,000
ASC 808 costs billed to Takeda						$ 195,000
2016 Restated Tokeda agreement
Collaboration agreements
Number of targets | target									7
Period of time until agreement terminated	45 days
2016 Restated Tokeda agreement and XMT-1522 agreement
Collaboration agreements
Amounts included in the contract liability at the beginning of the period			$ 39,965,000
Number of performance obligations | agreement						14	14
Collaboration revenue				0	0	$ 39,965,000
2016 Restated Tokeda agreement and XMT-1522 agreement | License and related research and development services, including phase 1 development and transfer certain materials and know how
Collaboration agreements
Number of performance obligations | agreement						1	1
Asana BioSciences
Collaboration agreements
Next potential milestone payment eligible to receive				2,500,000
Asana BioSciences | Limited services
Collaboration agreements
Collaboration revenue				0	0	$ 25,000
Merck KGaA supply agreement
Collaboration agreements
Collaboration revenue				$ 0	$ 0	$ 1,280,000