Represents the potential fee receivable for each license exercised under the collaborative agreement.

No definition available.

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

- 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:	instant

- Definition

Revenue from multiple-deliverable arrangements that include milestone and licensing fees revenue. Milestone revenue is consideration received upon achieving contractual goals. Licensing revenue is consideration received from another party for the right to use, but not own, certain of the entity's intangible assets.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of related targets.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of fee received for each license under the collaborative arrangement.

+ References

No definition available.

+ Details

Name:	imgn_ProceedsFromCollaboratorsPerLicense
Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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=31042434&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_SanofiMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	imgn_DevelopmentPhaseAxis=imgn_PhaseIClinicalTrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	imgn_DevelopmentPhaseAxis=imgn_PhaseIibClinicalTrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	imgn_MilestonePaymentsCategoryAxis=imgn_DevelopmentMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	imgn_MilestonePaymentsCategoryAxis=imgn_RegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=imgn_Sar650984Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=imgn_Sar408701Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaborative Agreements (Details 3) $ in Thousands	1 Months Ended		3 Months Ended		147 Months Ended
Collaborative Agreements (Details 3) $ in Thousands	Oct. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2006 USD ($)	Jul. 31, 2003 USD ($)
Collaborative Agreements disclosures
License and milestone fees			$ 6,070	$ 6,234
Sanofi \| Development and Commercialization License
Collaborative Agreements disclosures
Number of undisclosed targets with exclusive licenses \| item			1
Potential milestone payments						$ 30,000	$ 21,500
Payments received under collaboration agreement					$ 20,500
License exercise fee, per license						2,000
Fee received per license		$ 2,000
License and milestone fees				1,700
Sanofi \| Development and Commercialization License \| Phase I clinical trial \| Subsequent event
Collaborative Agreements disclosures
License and milestone fees	$ 2,000
Sanofi \| Development and Commercialization License \| Phase IIb clinical trial
Collaborative Agreements disclosures
Potential milestone payments			$ 4,000		$ 4,000
Sanofi \| Development and Commercialization License \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments						10,000	7,500
Sanofi \| Development and Commercialization License \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments						$ 20,000	$ 14,000
Sanofi \| Development and Commercialization License \| SAR650984 \| Phase IIb clinical trial
Collaborative Agreements disclosures
Payments received under collaboration agreement				3,000
Sanofi \| Development and Commercialization License \| SAR408701 \| Phase I clinical trial
Collaborative Agreements disclosures
Payments received under collaboration agreement				$ 1,000