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

+ References

No definition available.

+ Details

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

- 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 amount of potential payments that could be received under the collaborative agreement under the second option depending on the composition of any resulting products.

+ References

No definition available.

+ Details

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

- Definition

Represents the cumulative catch-up amount of revenue included in milestone and licensing fees revenue.

+ References

No definition available.

+ Details

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

- 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 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 received for extension of the collaborative agreement.

+ References

No definition available.

+ Details

Name:	imgn_ProceedsFromCollaboratorsContractExtension
Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the cash received from collaborators in connection with the amended agreement during the current period.

+ References

No definition available.

+ Details

Name:	imgn_ProceedsFromCollaboratorsInConnectionWithAmendedAgreement
Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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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

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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

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

- Definition

Cash received from licensees for license fees during the current period.

+ References

+ Details

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

- Definition

The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 985

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 730

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

+ Details

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

- Details

Name:	us-gaap_CounterpartyNameAxis=imgn_NovartisInstitutesForBioMedicalResearchIncMember
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Collaborative Agreements (Details 4) (USD $)	3 Months Ended		6 Months Ended		37 Months Ended	1 Months Ended				2 Months Ended	1 Months Ended		13 Months Ended
Collaborative Agreements (Details 4) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Oct. 31, 2013	Jan. 31, 2015	Oct. 31, 2013	Mar. 31, 2013	Oct. 31, 2010	Nov. 30, 2013 item	Oct. 31, 2014 item	Nov. 30, 2013	Oct. 31, 2014 item
Collaborative Agreements disclosures
Amount of arrangement consideration included in license and milestone fees	$ 41,417,000imgn_LicenseAndMilestoneFees	$ 25,678,000imgn_LicenseAndMilestoneFees	$ 47,651,000imgn_LicenseAndMilestoneFees	$ 38,845,000imgn_LicenseAndMilestoneFees
Costs related to the research and development services	27,647,000us-gaap_ResearchAndDevelopmentExpense	20,862,000us-gaap_ResearchAndDevelopmentExpense	55,665,000us-gaap_ResearchAndDevelopmentExpense	42,891,000us-gaap_ResearchAndDevelopmentExpense
Novartis
Collaborative Agreements disclosures
Payments received under collaboration agreement					60,200,000
Novartis \| LOP628 \| Subsequent event
Collaborative Agreements disclosures
Amount of arrangement consideration included in license and milestone fees						5,000,000
Novartis \| Right-to-test agreement
Collaborative Agreements disclosures
Term of agreement									3 years
Term of extension of the agreement							1 year
Payments for extension of agreement							5,000,000
Payments received under collaboration agreement								1,000,000	45,000,000
Payments received under collaboration agreement in connection with amended agreement								3,500,000
Novartis \| Right-to-test agreement \| Undisclosed Target
Collaborative Agreements disclosures
Number of pre-defined and related undisclosed targets								2
Novartis \| Right-to-test agreement \| Exclusive license
Collaborative Agreements disclosures
Number of development and commercialization licenses taken									6
Novartis \| Right-to-test agreement \| Exclusive license \| Undisclosed Target
Collaborative Agreements disclosures
Number of pre-defined and related undisclosed targets								1
Novartis \| Development and Commercialization License
Collaborative Agreements disclosures
Payments received under collaboration agreement					55,000,000
License exercise fee, per license									1,000,000
Potential milestone payments								199,500,000	199,500,000
Number of development and commercialization licenses taken										2
Potential milestone payments under second option								238,000,000
Amount of arrangement consideration included in license and milestone fees	25,700,000	17,200,000	25,700,000	17,200,000
Cumulative catch-up amount included in license and milestone fees		1,000,000		1,000,000
Estimated utilization period after commercialization			10 years
Discount rate (as a percent)			16.00%
Estimated term of development and commercialization license			25 years
Novartis \| Development and Commercialization License \| Phase I clinical trial \| LOP628 \| Subsequent event
Collaborative Agreements disclosures
Proceeds from License Fees Received						5,000,000
Novartis \| Development and Commercialization License \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments								22,500,000	22,500,000
Potential milestone payments under second option								22,500,000
Novartis \| Development and Commercialization License \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments								77,000,000	77,000,000
Potential milestone payments under second option								115,500,000
Novartis \| Development and Commercialization License \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments								100,000,000	100,000,000
Potential milestone payments under second option								100,000,000
Novartis \| Development and Commercialization License \| Future Technological Improvements
Collaborative Agreements disclosures
Payments received under collaboration agreement					4,500,000
Novartis \| Development and Commercialization License \| Research Services
Collaborative Agreements disclosures
Payments received under collaboration agreement					710,000
Novartis \| Development and Commercialization License \| Exclusive license
Collaborative Agreements disclosures
Payments received under collaboration agreement							1,000,000				1,000,000	1,000,000
Potential milestone payments					199,500,000		199,500,000			199,500,000	199,500,000	199,500,000	199,500,000
Number of development and commercialization licenses taken			5								3		5
Number of pre-defined and related undisclosed targets			2
Novartis \| Development and Commercialization License \| Exclusive license \| Phase I clinical trial
Collaborative Agreements disclosures
Potential milestone payments	5,000,000		5,000,000
Novartis \| Development and Commercialization License \| Exclusive license \| Phase II clinical trial
Collaborative Agreements disclosures
Potential milestone payments	$ 7,500,000		$ 7,500,000