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
Period Type:	duration

- Definition

Represents the number of licenses terminated.

+ References

No definition available.

+ Details

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Balance Type:	na
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- 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
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- Definition

Represents the amount of fee received for each license under the collaborative arrangement.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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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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

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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

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Details

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- Details

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- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_RegulatoryMilestoneInvestigationalNewDrugApplicationFilingMember
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Data Type:	na
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Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_PhaseIIClinicalTrialMember
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- Details

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- Details

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Collaborative Agreements - Amgen (Details)	1 Months Ended					3 Months Ended		12 Months Ended	201 Months Ended
Collaborative Agreements - Amgen (Details)	Feb. 29, 2016 item	Dec. 31, 2015 item	Sep. 30, 2015 USD ($)	Oct. 31, 2013 USD ($)	May 31, 2013 USD ($) item	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Dec. 31, 2000 USD ($) item	Sep. 30, 2016 USD ($)
Collaborative Agreements disclosures
Amount of arrangement consideration included in license and milestone fees						$ 76,000	$ 6,070,000
Costs related to the research and development services						$ 32,909,000	$ 35,132,000
Amgen
Collaborative Agreements disclosures
Number of single-target licenses \| item					1	2		3
Fee received per license								$ 1,000,000
Payments received under collaboration agreement			$ 1,000,000	$ 500,000	$ 500,000				$ 3,000,000
Number of licenses terminated \| item	2	2
Potential milestone payments								34,000,000
Amgen \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments								9,000,000
Amgen \| IND application filed
Collaborative Agreements disclosures
Potential milestone payments						$ 1,000,000			1,000,000
Amgen \| Phase II clinical trial
Collaborative Agreements disclosures
Potential milestone payments						$ 3,000,000			$ 3,000,000
Amgen \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments								20,000,000
Amgen \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments								$ 5,000,000