Represents the amount of potential payments that could be received under the collaborative agreement.

No definition available.

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- Definition

Revenue from multiple-deliverable arrangements that include milestone and licensing fees revenue. Milestone revenue is consideration received upon achieving contractual goals. Licensing revenue is consideration received from another party for the right to use, but not own, certain of the entity's intangible assets.

+ References

No definition available.

+ Details

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Balance Type:	credit
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Balance Type:	na
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- Definition

Cash received from collaborators during the current period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

+ Details

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Period Type:	duration

- Definition

The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Namespace Prefix:
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- Details

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- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_PhaseIClinicalTrialMember
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- Details

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- Details

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Data Type:	na
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Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_SalesMilestonesMember
Namespace Prefix:
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Agreements - CytomX (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
Agreements - CytomX (Details) - USD ($) $ in Thousands	Feb. 29, 2016	Jan. 31, 2014	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Collaborative Agreements disclosures
License and milestone fees			$ 31,080	$ 76	$ 49,810	$ 10,153
Costs related to the research and development services			35,319	$ 38,652	68,207	$ 74,746
CytomX
Collaborative Agreements disclosures
Payments received under collaboration agreement	$ 13,000	$ 0
License and milestone fees					12,700
Potential milestone payments	160,000
CytomX \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments	10,000
CytomX \| Phase 1 clinical trial
Collaborative Agreements disclosures
License and milestone fees			1,000		1,000
CytomX \| Phase 2 clinical trial
Collaborative Agreements disclosures
Potential milestone payments			$ 3,000		$ 3,000
CytomX \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments	50,000
CytomX \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments	$ 100,000