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

No definition available.

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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

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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

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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

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Namespace Prefix:	imgn_
Data Type:	xbrli:integerItemType
Balance Type:	na
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

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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:
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Balance Type:
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- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_RightToTestMember
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_ExerciseFeePerLicenseAxis=imgn_ExerciseFeeMember
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_TypeOfLicenseAxis=imgn_ExclusiveDevelopmentAndCommercializationLicenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
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- Details

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Namespace Prefix:
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- Details

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- Details

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Namespace Prefix:
Data Type:	na
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- Details

Name:	imgn_MilestonePaymentsCategoryAxis=imgn_RegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	imgn_MilestonePaymentsCategoryAxis=imgn_SalesMilestonesMember
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Collaborative Agreements - Lilly (Details) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended		37 Months Ended
Collaborative Agreements - Lilly (Details) $ in Thousands	Sep. 30, 2015 USD ($)	Dec. 31, 2011 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Collaborative Agreements disclosures
Amount of arrangement consideration included in license and milestone fees			$ 10,692	$ 41,417	$ 16,762	$ 47,651
Lilly \| Right-to-test agreement
Collaborative Agreements disclosures
Number of development and commercialization licenses taken \| item		3
Payments received under collaboration agreement		$ 20,000
Lilly \| Right-to-test agreement \| No exercise fee
Collaborative Agreements disclosures
License exercise fee, per subsequent license		0
Lilly \| Right-to-test agreement \| Exercise fee of $2 million
Collaborative Agreements disclosures
License exercise fee, per subsequent license		$ 2,000						$ 2,000
Lilly \| Development and Commercialization License
Collaborative Agreements disclosures
Payments received under collaboration agreement							$ 23,500
Amount of arrangement consideration included in license and milestone fees				15,600		15,600
Lilly \| Development and Commercialization License \| Exclusive license
Collaborative Agreements disclosures
Number of development and commercialization licenses taken \| item					3
Lilly \| Development and Commercialization License \| No exercise fee
Collaborative Agreements disclosures
Number of development and commercialization licenses taken \| item					1
Lilly \| Development and Commercialization License \| No exercise fee \| Maximum
Collaborative Agreements disclosures
Potential milestone payments for each subsequent license				200,500		200,500	200,500
Lilly \| Development and Commercialization License \| Exercise fee of $2 million
Collaborative Agreements disclosures
Number of development and commercialization licenses taken \| item		2			2
License exercise fee, per subsequent license				2,000		2,000	2,000
Lilly \| Development and Commercialization License \| Exercise fee of $2 million \| Maximum
Collaborative Agreements disclosures
Potential milestone payments for each subsequent license				199,000		199,000	199,000
Lilly \| Development and Commercialization License \| Phase I clinical trial
Collaborative Agreements disclosures
Number of development and commercialization licenses taken \| item					2
Lilly \| Development and Commercialization License \| Development milestones \| No exercise fee
Collaborative Agreements disclosures
Potential milestone payments for each subsequent license				30,500		30,500	30,500
Lilly \| Development and Commercialization License \| Development milestones \| Exercise fee of $2 million
Collaborative Agreements disclosures
Potential milestone payments for each subsequent license				29,000		29,000	29,000
Lilly \| Development and Commercialization License \| Development milestones \| Phase I clinical trial
Collaborative Agreements disclosures
Payments received under collaboration agreement	$ 5,000
Potential milestone payments for each subsequent license			$ 5,000		$ 5,000
Lilly \| Development and Commercialization License \| Development milestones \| Phase II clinical trial
Collaborative Agreements disclosures
Potential milestone payments for each subsequent license	$ 9,000
Lilly \| Development and Commercialization License \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments for each subsequent license				70,000		70,000	70,000
Lilly \| Development and Commercialization License \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments for each subsequent license				$ 100,000		$ 100,000	$ 100,000