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 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, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Indicates type of revenue from product and service. Includes, but is not limited to, revenue from contract with customer and other sources.

+ References

No definition available.

+ Details

Name:	us-gaap_TypeOfRevenueExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	srt-types:extensibleListItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_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:

- Details

Name:	srt_ProductOrServiceAxis=imgn_LicenseAndMilestoneFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Agreements - Takeda (Details) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
Agreements - Takeda (Details) $ in Thousands	May 31, 2018 USD ($)	Mar. 31, 2015 item	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)
Collaborative Agreements disclosures
Revenue from contract with customer			$ 13,281	$ 10,928	$ 37,407	$ 40,030
Takeda
Collaborative Agreements disclosures
Term of agreement		3 years
Number of single-target licenses \| item		2			1
Revenue from contract with customer						$ 10,900
Type of Revenue						imgn:LicenseAndMilestoneFeesMember
Takeda \| Phase 1 clinical trial
Collaborative Agreements disclosures
Revenue from contract with customer	$ 5,000
Takeda \| Phase 2 clinical trial
Collaborative Agreements disclosures
Revenue from contract with customer	$ 10,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer			$ 79	$ 672	$ 5,237	$ 13,533