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 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 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 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

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

- Details

Name:	us-gaap_CounterpartyNameAxis=imgn_TakedaOncologyMember
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_TypeOfArrangementAxis=imgn_RightToTestMember
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_MilestonePaymentsCategoryAxis=imgn_OptionToExpandAgreementMember
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_ExerciseFeePerLicenseAxis=imgn_NoExerciseFeeMember
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:	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:

Collaborative Agreements - Takeda (Details) $ in Thousands	3 Months Ended			6 Months Ended		10 Months Ended
Collaborative Agreements - Takeda (Details) $ in Thousands	Dec. 31, 2015 USD ($)	Mar. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)
Collaborative Agreements disclosures
Amount of arrangement consideration included in license and milestone fees	$ 10,692		$ 41,417	$ 16,762	$ 47,651
Takeda \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments	30,000			30,000		$ 30,000
Takeda \| Right-to-test agreement
Collaborative Agreements disclosures
Term of agreement		3 years
Term of extension of the agreement		1 year
Payments to be made for extension of agreement		$ 4,000
Payments received under collaboration agreement		$ 20,000
Takeda \| Right-to-test agreement \| Maximum
Collaborative Agreements disclosures
Number of single-target licenses \| item		2
Takeda \| Right-to-test agreement \| Option To Expand Agreement
Collaborative Agreements disclosures
Payments to be made for extension of agreement		$ 8,000
Takeda \| Development and Commercialization License
Collaborative Agreements disclosures
Payments received under collaboration agreement						25,900
Amount of arrangement consideration included in license and milestone fees	8,600			8,600
Takeda \| Development and Commercialization License \| No exercise fee \| Maximum
Collaborative Agreements disclosures
Potential milestone payments	210,000			210,000		210,000
Takeda \| Development and Commercialization License \| Phase I clinical trial
Collaborative Agreements disclosures
Potential milestone payments	5,000			5,000		5,000
Takeda \| Development and Commercialization License \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments	85,000			85,000		85,000
Takeda \| Development and Commercialization License \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments	$ 95,000			$ 95,000		$ 95,000