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=56944662&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:

Collaborative Agreements - Roche (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended	201 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2000	Sep. 30, 2016
Collaborative Agreements disclosures
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	$ 6,184	$ 5,684
Roche \| Kadcyla
Collaborative Agreements disclosures
Payments received under collaboration agreement			$ 2,000
Potential milestone payments			44,000
Non-cash royalty revenue related to sale of future royalties	6,200	$ 5,700
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		$ 30,500	$ 5,000