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:	instant

- Definition

Represents the number of milestone payments received.

+ References

No definition available.

+ Details

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

- 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 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=56944662&loc=d3e3536-108585

+ Details

Name:	us-gaap_ProceedsFromCollaborators
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Revenue earned during the period from the leasing or otherwise lending to a third party the entity's rights or title to certain property. Royalty revenue is derived from a percentage or stated amount of sales proceeds or revenue generated by the third party using the entity's property. Examples of property from which royalties may be derived include patents and oil and mineral rights.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

Agreements - Roche (Details) $ in Thousands	3 Months Ended								12 Months Ended				198 Months Ended
Agreements - Roche (Details) $ in Thousands	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($) item	Dec. 31, 2000 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2006 USD ($)
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties	$ 5,944	$ 7,380	$ 6,291	$ 5,684	$ 5,484				$ 25,299	$ 5,484
Royalties on net sales of Kadcyla			$ 195		$ (23)	$ 5,099	$ 4,625	$ 4,166	195	13,867	$ 10,346
Development and Commercialization License \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments														$ 4,500
Roche \| Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties									25,300	5,500
Royalties on net sales of Kadcyla										$ 13,900	$ 10,300
Roche \| Development and Commercialization License \| Kadcyla
Collaborative Agreements disclosures
Payments received under collaboration agreement												$ 2,000
Potential milestone payments												44,000
Roche \| Development and Commercialization License \| Kadcyla \| Development milestones
Collaborative Agreements disclosures
Payments received under collaboration agreement													$ 13,500
Potential milestone payments												13,500
Roche \| Development and Commercialization License \| Kadcyla \| Regulatory milestones
Collaborative Agreements disclosures
Payments received under collaboration agreement													20,500
Potential milestone payments	5,000								5,000			30,500	5,000
Number of milestone payments received \| item											2
Roche \| Development and Commercialization License \| Kadcyla \| Regulatory milestones in Japan
Collaborative Agreements disclosures
Payments received under collaboration agreement											$ 5,000
Roche \| Development and Commercialization License \| Kadcyla \| Regulatory milestones in Europe
Collaborative Agreements disclosures
Payments received under collaboration agreement											$ 5,000
Roche \| Right-to-test agreement \| Undisclosed Target
Collaborative Agreements disclosures
Payments received under collaboration agreement												5,000
Potential milestone payments												$ 38,000
Number of undisclosed targets with exclusive licenses \| item												4
License exercise fee												$ 1,000
Term of agreement												8 years
Roche \| Right-to-test agreement \| Undisclosed Target \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments												$ 8,000
Roche \| Right-to-test agreement \| Undisclosed Target \| IND application filed
Collaborative Agreements disclosures
Potential milestone payments	$ 1,000								$ 1,000				$ 1,000
Roche \| Right-to-test agreement \| Undisclosed Target \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments												20,000
Roche \| Right-to-test agreement \| Undisclosed Target \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments												$ 10,000