Represents the potential fee receivable for each license exercised under the collaborative agreement.

No definition available.

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Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
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- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the amount of potential payments that could be received under the collaborative agreement with a no exercise fee option.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of single-target licenses the company has right to use.

+ References

No definition available.

+ Details

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Namespace Prefix:	imgn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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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Data Type:	xbrli:stringItemType
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=56944662&loc=d3e3536-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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- Details

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- Details

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- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_DevelopmentMilestonesMember
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- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_PhaseIClinicalTrialMember
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- Details

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Collaborative Agreements - Lily (Details) $ in Thousands	1 Months Ended		3 Months Ended		12 Months Ended
Collaborative Agreements - Lily (Details) $ in Thousands	Sep. 30, 2015 USD ($)	Dec. 31, 2013 USD ($) item	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Dec. 31, 2011 USD ($) item
Collaborative Agreements disclosures
Amount of arrangement consideration included in license and milestone fees			$ 76	$ 6,070
Costs related to the research and development services			$ 32,909	$ 35,132
Lilly
Collaborative Agreements disclosures
Number of development and commercialization licenses taken \| item			3		3
Payments received under collaboration agreement	$ 5,000	$ 2,000			$ 20,000
Lilly \| No exercise fee
Collaborative Agreements disclosures
Number of development and commercialization licenses taken \| item		1
Potential milestone payments with no exercise fee					200,500
Lilly \| Exercise fee
Collaborative Agreements disclosures
Number of development and commercialization licenses taken \| item		1
License exercise fee, per license					2,000
Potential milestone payments					199,000
Lilly \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments					29,000
Potential milestone payments with no exercise fee					30,500
Lilly \| Phase I clinical trial
Collaborative Agreements disclosures
Potential milestone payments			$ 5,000
Lilly \| Phase II clinical trial
Collaborative Agreements disclosures
Potential milestone payments			$ 9,000
Lilly \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments					70,000
Lilly \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments					$ 100,000