Represents the amount of potential payments that could be received under the collaborative agreement.

No definition available.

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

- Definition

Represents the number of compounds under development and commercialization licenses under the collaborative agreements entered into by the entity.

+ References

No definition available.

+ Details

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

- 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

Amount of cumulative earnings (deficits) for relevant time periods.

+ References

No definition available.

+ Details

Name:	us-gaap_CumulativeEarningsDeficit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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=98514028&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:

- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_DevelopmentMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=imgn_RegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Agreements - Sanofi (Details) $ in Thousands	1 Months Ended		3 Months Ended														6 Months Ended		12 Months Ended
Agreements - Sanofi (Details) $ in Thousands	May 31, 2017 USD ($) item	Dec. 31, 2013 USD ($) item	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2003 USD ($)
Collaborative Agreements disclosures
License and milestone fees			$ 29,580	$ 79	$ 31,080	$ 18,730	$ 5,076	$ 76	$ 76	$ 10,077	$ 10,692	$ 6,070	$ 5,086	$ 5,078	$ 41,417	$ 6,234	$ 5,152	$ 16,762	$ 79,469	$ 15,305	$ 26,915	$ 57,815
Sanofi
Collaborative Agreements disclosures
Potential milestone payments																							$ 21,500
Number of compounds \| item	4	1
Payments received under collaboration agreement	$ 30,000																		6,000			4,000
Cumulative earnings			$ 26,500																26,500
Fee received per license		$ 2,000
License and milestone fees																			$ 30,000			$ 1,500
Sanofi \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments																							7,500
Sanofi \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments																							$ 14,000