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

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 increase (decrease) in right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, from business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130551-203045

+ Details

Name:	us-gaap_ContractWithCustomerAssetIncreaseDecreaseForContractAcquiredInBusinessCombination
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

- Details

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

- Details

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

- Details

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

Agreements - Takeda (Details) $ in Thousands	1 Months Ended		6 Months Ended
Agreements - Takeda (Details) $ in Thousands	May 31, 2018 USD ($)	Mar. 31, 2015 item	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Collaborative Agreements disclosures
Arrangement consideration included in long-term deferred revenue			$ 80,751	$ 93,752
Milestone recognized as contract asset			$ 4,041
Takeda
Collaborative Agreements disclosures
Term of agreement		3 years
Number of single-target licenses \| item		2	1
Arrangement consideration included in long-term deferred revenue			$ 10,900
Takeda \| Phase 1 clinical trial
Collaborative Agreements disclosures
Milestone recognized as contract asset	$ 5,000
Takeda \| Phase 2 clinical trial
Collaborative Agreements disclosures
Potential milestone payment			$ 10,000