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

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

Total costs related to goods produced and sold during the reporting period.

+ 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 210.5-03.2(a))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 2

-Article 5

+ Details

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

- Definition

Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response)

-URI http://asc.fasb.org/extlink&oid=27012821&loc=d3e214044-122780

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 13

-Section A

+ Details

Name:	us-gaap_DeferredRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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:	imgn_CollaborativeArrangementsAndNoncollaborativeArrangementsTransactionsAxis=imgn_BayerHealthCareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Agreements (Details 5) - Bayer HealthCare - USD ($)	1 Months Ended	12 Months Ended			81 Months Ended
Agreements (Details 5) - Bayer HealthCare - USD ($)	Oct. 31, 2008	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2015
Collaborative Agreements disclosures
Costs related to clinical materials sold		$ 0	$ 0	$ 297,000
Exclusive license
Collaborative Agreements disclosures
Payments received under collaboration agreement	$ 4,000,000				$ 3,000,000
Exclusive license \| Maximum
Collaborative Agreements disclosures
Potential milestone payments	170,500,000
Exclusive license \| Phase II clinical trial
Collaborative Agreements disclosures
Potential milestone payments		4,000,000			4,000,000
Deferred upfront payment		$ 4,000,000			$ 4,000,000
Exclusive license \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments	16,000,000
Exclusive license \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments	44,500,000
Exclusive license \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments	$ 110,000,000