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

No definition available.

Name:	imgn_CollaborativeArrangementExerciseFeePotentialPerLicense
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.

+ 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 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

- 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=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:	us-gaap_CounterpartyNameAxis=imgn_SanofiMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaborative Agreements (Details 3) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended
Collaborative Agreements (Details 3) (USD $)	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2006	Jul. 31, 2003
Collaborative Agreements disclosures
License and milestone fees	$ 5,078,000imgn_LicenseAndMilestoneFees	$ 305,000imgn_LicenseAndMilestoneFees	$ 52,729,000imgn_LicenseAndMilestoneFees	$ 39,150,000imgn_LicenseAndMilestoneFees
Sanofi \| Development and Commercialization License
Collaborative Agreements disclosures
Potential milestone payments	2,000,000		2,000,000			30,000,000	21,500,000
License exercise fee, per license						2,000,000
Fee received per license					2,000,000
Payments received under collaboration agreement			20,500,000
License and milestone fees			1,600,000
Sanofi \| Development and Commercialization License \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments	2,000,000		2,000,000			10,000,000	7,500,000
Sanofi \| Development and Commercialization License \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments						20,000,000	14,000,000
Sanofi \| Development and Commercialization License \| SAR650984 \| Development milestones \| Phase IIb Clinical Trial
Collaborative Agreements disclosures
Payments received under collaboration agreement			3,000,000
Sanofi \| Development and Commercialization License \| SAR650984 \| Development milestones \| Phase III Clinical Trial
Collaborative Agreements disclosures
Potential milestone payments	3,000,000		3,000,000
Sanofi \| Development and Commercialization License \| SAR566658 \| Development milestones \| Phase IIb Clinical Trial
Collaborative Agreements disclosures
Potential milestone payments	3,000,000		3,000,000
Sanofi \| Development and Commercialization License \| SAR408701 \| Development milestones \| Phase IIb Clinical Trial
Collaborative Agreements disclosures
Potential milestone payments	3,000,000		3,000,000
Sanofi \| Development and Commercialization License \| SAR408701 \| Development milestones \| Phase I clinical trial
Collaborative Agreements disclosures
Payments received under collaboration agreement			1,000,000
Sanofi \| Development and Commercialization License \| Undisclosed Target \| Development milestones \| Phase I clinical trial
Collaborative Agreements disclosures
Potential milestone payments	$ 1,000,000		$ 1,000,000