Represents the initial period of the collaborative agreement.

No definition available.

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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

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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

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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

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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 related targets.

+ References

No definition available.

+ Details

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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

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

- Definition

Represents the amount of fee received for each license under the collaborative arrangement.

+ References

No definition available.

+ Details

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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

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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

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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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_DevelopmentPhaseAxis=imgn_PhaseIClinicalTrialMember
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:	us-gaap_ProductOrServiceAxis=imgn_IsatuximabMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	imgn_DevelopmentPhaseAxis=imgn_PhaseIibClinicalTrialMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_ProductOrServiceAxis=imgn_Sar408701Member
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Data Type:	na
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- Details

Name:	imgn_TypeOfLicenseAxis=imgn_ExclusiveDevelopmentAndCommercializationLicenseMember
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Data Type:	na
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- Details

Name:	us-gaap_ProductOrServiceAxis=imgn_TargetUndisclosedMember
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Data Type:	na
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- Details

Name:	us-gaap_ProductOrServiceAxis=imgn_Sar566658Member
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- Details

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Agreements (Details 3) - Range [Domain]	1 Months Ended				3 Months Ended								12 Months Ended			21 Months Ended	144 Months Ended
Agreements (Details 3) - Range [Domain]	Dec. 31, 2013 USD ($)	Aug. 31, 2011 USD ($)	Aug. 31, 2008 USD ($)	Dec. 31, 2006 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2015 USD ($) item	Jun. 30, 2014 USD ($)	Jun. 30, 2013 USD ($)	Aug. 31, 2008 USD ($)	Jun. 30, 2015 USD ($)	Jul. 31, 2003 USD ($)
Collaborative Agreements disclosures
License and milestone fees					$ 5,086,000	$ 5,078,000	$ 41,417,000	$ 6,234,000	$ 305,000	$ 305,000	$ 25,678,000	$ 13,167,000	$ 57,815,000	$ 39,455,000	$ 24,227,000
Sanofi \| Right-to-test agreement
Collaborative Agreements disclosures
Payments received under collaboration agreement			$ 3,500,000	$ 500,000												$ 4,000,000
Term of agreement				3 years
Term of extension of the agreement		3 years
Payments for extension of agreement		$ 2,000,000
Sanofi \| Development and Commercialization License
Collaborative Agreements disclosures
Number of undisclosed targets with exclusive licenses \| item													1
Potential milestone payments				$ 30,000,000														$ 21,500,000
Payments received under collaboration agreement																	$ 20,500,000
License exercise fee, per license				2,000,000
Fee received per license	$ 2,000,000
License and milestone fees													$ 1,500,000
Sanofi \| Development and Commercialization License \| Phase I clinical trial
Collaborative Agreements disclosures
Potential milestone payments					2,000,000								2,000,000				2,000,000
Sanofi \| Development and Commercialization License \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments				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 \| Isatuximab \| Development milestones \| Phase IIb clinical trial
Collaborative Agreements disclosures
Payments received under collaboration agreement													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 \| Exclusive license \| Undisclosed Target
Collaborative Agreements disclosures
Payments received under collaboration agreement															$ 500,000
Sanofi \| Development and Commercialization License \| Exclusive license \| Undisclosed Target \| Phase I clinical trial
Collaborative Agreements disclosures
Potential milestone payments					1,000,000								1,000,000				1,000,000
Sanofi \| Development and Commercialization License \| Exclusive license \| SAR566658 \| Development milestones \| Phase IIb clinical trial
Collaborative Agreements disclosures
Potential milestone payments					3,000,000								3,000,000				3,000,000
Sanofi \| Development and Commercialization License \| Exclusive license \| Isatuximab \| Development milestones \| Phase III clinical trial
Collaborative Agreements disclosures
Potential milestone payments					$ 3,000,000								$ 3,000,000				$ 3,000,000