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

No definition available.

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

- Definition

Revenue earned from sale of future royalties during the period from the leasing or otherwise lending to a third party the entity's rights or title to certain property

+ References

No definition available.

+ Details

Name:	imgn_NonCashRoyaltyRevenueRelatedToSaleOfFutureRoyalties
Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the period in arrears to receive royalty reports and payments related to sales of Kadcyla.

+ References

No definition available.

+ Details

Name:	imgn_PeriodInArrearsToReceiveRoyaltyReportsAndPayments
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
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

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

- Details

Name:	us-gaap_ProductOrServiceAxis=imgn_KadcylaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Agreements - Roche (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended	210 Months Ended
Agreements - Roche (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2000	Jun. 30, 2017
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties	$ 6,439	$ 5,944	$ 14,052	$ 13,324
Roche \| Kadcyla
Collaborative Agreements disclosures
Payments received under collaboration agreement					$ 2,000
Potential milestone payments					$ 44,000
Period in arrears to receive royalty reports and payments related to sales of Kadcyla					3 months
Non-cash royalty revenue related to sale of future royalties			14,100	$ 13,300
Roche \| Kadcyla \| Development milestones
Collaborative Agreements disclosures
Payments received under collaboration agreement						$ 13,500
Potential milestone payments					$ 13,500
Roche \| Kadcyla \| Regulatory milestones
Collaborative Agreements disclosures
Payments received under collaboration agreement						20,500
Potential milestone payments	$ 5,000		$ 5,000		$ 30,500	$ 5,000