Represents the initial period of the collaborative agreement.

No definition available.

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- Definition

The Company?s development and commercialization license agreements have milestone payments which for reporting purposes are aggregated into three categories: (i) development milestones, (ii) regulatory milestones, and (iii) sales milestones. Development milestones are typically payable when a product candidate initiates or advances into different clinical trial phases. Regulatory milestones are typically payable upon submission for marketing approval with the U.S. Food and Drug Administration, or FDA, or other countries? regulatory authorities or on receipt of actual marketing approvals for the compound or for additional indications. Sales milestones are typically payable when annual sales reach certain levels.

+ References

No definition available.

+ Details

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

- 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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Namespace Prefix:	imgn_
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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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
Period Type:	duration

- Definition

Amount, before allowance for credit loss, of right to consideration in exchange for good or service transferred to customer, when right is conditioned on something other than passage of time.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130533-203044

+ Details

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

- Details

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Namespace Prefix:
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- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_PhaseIIClinicalTrialMember
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- Details

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Data Type:	na
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- Details

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Data Type:	na
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_SalesMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201409Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InitialApplicationPeriodCumulativeEffectTransitionAxis=us-gaap_DifferenceBetweenRevenueGuidanceInEffectBeforeAndAfterTopic606Member
Namespace Prefix:
Data Type:	na
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Agreements - Fusion (Details)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
Agreements - Fusion (Details)	Dec. 31, 2016 USD ($)	Mar. 31, 2015 item	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	May 31, 2018 USD ($)
Collaborative Agreements disclosures
License and milestone fees			$ 672,000	$ 79,000	$ 13,533,000	$ 49,889,000
Takeda
Collaborative Agreements disclosures
Term of agreement		3 years
Number of single-target licenses \| item		2			1
License and milestone fees					$ 10,900,000
Takeda \| Phase 2 clinical trial
Collaborative Agreements disclosures
Potential milestone payment					$ 10,000,000
Fusion Pharmaceuticals
Collaborative Agreements disclosures
Number of single-target licenses \| item					1
Potential milestone payment	$ 50,000,000
Fusion Pharmaceuticals \| Development milestones
Collaborative Agreements disclosures
Potential milestone payment	15,000,000
Fusion Pharmaceuticals \| Phase 1 clinical trial
Collaborative Agreements disclosures
Contract asset recorded for the probable milestone	500,000
Fusion Pharmaceuticals \| Phase 2 clinical trial
Collaborative Agreements disclosures
Contract asset recorded for the probable milestone	1,500,000
Fusion Pharmaceuticals \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payment	$ 35,000,000
ASU 2014-09 \| Adjustments due to new guidance
Collaborative Agreements disclosures
Potential milestone payment			$ 500,000		$ 5,000,000
ASU 2014-09 \| Adjustments due to new guidance \| Takeda \| Phase 1 clinical trial
Collaborative Agreements disclosures
Contract asset recorded for the probable milestone								$ 5,000,000
Potential milestone payment							$ 5,000,000