Collaborative Agreements (Details 7) (Takeda, USD $)	1 Months Ended	9 Months Ended
Mar. 31, 2015 item	Mar. 31, 2015 item
Collaborative Agreements disclosures
Payments received under collaboration agreement	$ 31,400,000us-gaap_ProceedsFromCollaborators
Right-to-test agreement
Collaborative Agreements disclosures
Number of single-target licenses	2
Term of agreement	3 years
Term of extension of the agreement	1 year
Payments to be made for extension of agreement	4,000,000
Payments received under collaboration agreement	20,000,000
Right-to-test agreement \| Maximum
Collaborative Agreements disclosures
Potential milestone payments	210,000,000	210,000,000
Right-to-test agreement \| Development milestones \| Maximum
Collaborative Agreements disclosures
Potential milestone payments	30,000,000	30,000,000
Right-to-test agreement \| Regulatory milestones \| Maximum
Collaborative Agreements disclosures
Potential milestone payments	85,000,000	85,000,000
Right-to-test agreement \| Sales milestones \| Maximum
Collaborative Agreements disclosures
Potential milestone payments	95,000,000	95,000,000
Right-to-test agreement \| Option To Expand Agreement
Collaborative Agreements disclosures
Payments to be made for extension of agreement	8,000,000
Development and Commercialization License
Collaborative Agreements disclosures
Discount rate (as a percent)	13.00%
Estimated utilization period after commercialization	10 years
Estimated term of development and commercialization license	25 years
Development and Commercialization License \| Phase I clinical trial
Collaborative Agreements disclosures
Potential milestone payments	5,000,000	5,000,000
Development and Commercialization License \| Option To Expand Agreement
Collaborative Agreements disclosures
Number of single-target licenses	1
Value of collaborators licenses after extension	8,000,000	8,000,000
Additional term of extension of the agreement		2 years
Number of development and commercialization licenses		2
Development and Commercialization License \| Option to expand agreement for one additional year
Collaborative Agreements disclosures
Term of extension of the agreement		1 year
Payments to be made for extension of agreement		$ 4,000,000

Represents the additional term of extension of the collaborative agreement.

No definition available.

Name:	imgn_CollaborativeArrangementAdditionalExtensionTerm
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the initial period of the collaborative agreement.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementAgreementTerm
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the estimated utilization period after commercialization of the product, which is used to allocate fees to the deliverables in the contract.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementAllocationOfDeliverablesAssumptionsUtilizationPeriod
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the term of extension of the collaborative agreement.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementExtensionTerm
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the estimated term of licenses under the collaborative arrangements.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementLicensesEstimatedTerm
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 development and commercialization licenses.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of single-target licenses the company has right to use.

+ References

No definition available.

+ Details

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

- Definition

Represents the payments to be made for extension of the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents information pertaining to value of collaborators licenses after extension.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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