Represents the initial period of the collaborative agreement.

No definition available.

Name:	imgn_CollaborativeArrangementAgreementTerm
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementMilestonePaymentsPotential
Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of hematology-related antibody-drug conjugate (ADC) programs under the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

The number of early stage hematology-related antibody-drug conjugate (ADC) programs under the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

The number of product for which there is right to co-commercialize in the U.S. under the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

The amount of reimbursement received under collaborative arrangement.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementReimbursementReceived
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

Name:	imgn_NumberOfSingleTargetLicenses
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

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as noncurrent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130549-203045

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130532-203044

+ Details

Name:	us-gaap_ContractWithCustomerLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

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

- Details

Name:	us-gaap_CounterpartyNameAxis=imgn_JazzPharmaceuticalsIrelandLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Agreements - Jazz Pharmaceuticals (Details) $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
Agreements - Jazz Pharmaceuticals (Details) $ in Thousands	Aug. 31, 2017 USD ($) product item	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Agreements disclosures
Remaining arrangement consideration to be recognized as license revenue		$ 80,751	$ 80,751	$ 93,752
Jazz Pharmaceuticals
Collaborative Agreements disclosures
Number of early stage ADC programs \| item	2
Number of ADC programs \| item	3
Payments received under collaboration agreement	$ 75,000
Term of agreement	7 years
Number of products with rights to co-commercialize \| product	1
Offset to research and development expense		1,800	3,800
Number of development and commercialization licenses \| item	3
Remaining arrangement consideration to be recognized as license revenue	$ 75,000	$ 75,000	$ 75,000
Jazz Pharmaceuticals \| Maximum
Collaborative Agreements disclosures
Potential milestone payment	$ 100,000