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 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 earned from sale of future royalties during the period from the leasing or otherwise lending to a third party the entity's rights or title to certain property

+ References

No definition available.

+ Details

Name:	imgn_NonCashRoyaltyRevenueRelatedToSaleOfFutureRoyalties
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

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

- Definition

Revenue earned during the period from the leasing or otherwise lending to a third party the entity's rights or title to certain property. Royalty revenue is derived from a percentage or stated amount of sales proceeds or revenue generated by the third party using the entity's property. Examples of property from which royalties may be derived include patents and oil and mineral rights.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_RoyaltyRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_ProductOrServiceAxis=imgn_KadcylaMember
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:

- Details

Name:	us-gaap_ProductOrServiceAxis=imgn_TargetUndisclosedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Agreements - Roche (Details) $ in Thousands	3 Months Ended														6 Months Ended		12 Months Ended
Agreements - Roche (Details) $ in Thousands	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, 2000 USD ($) item
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties	$ 7,587	$ 6,503	$ 6,439	$ 7,613	$ 6,710	$ 6,184	$ 5,944	$ 7,380	$ 6,291	$ 5,684	$ 5,484				$ 12,894	$ 11,975	$ 28,142	$ 26,218	$ 25,299	$ 5,484
Royalty revenue									$ 195			$ 5,099	$ 4,625	$ 4,166		$ 195			195	13,867
Roche \| Kadcyla
Collaborative Agreements disclosures
Payments received under collaboration agreement																					$ 2,000
Potential milestone payments																					44,000
Non-cash royalty revenue related to sale of future royalties															$ 12,900		28,100		$ 25,300	$ 5,500
Roche \| Kadcyla \| Development milestones
Collaborative Agreements disclosures
Cumulative earnings	13,500																13,500
Potential milestone payments																					13,500
Roche \| Kadcyla \| Regulatory milestones
Collaborative Agreements disclosures
Cumulative earnings	20,500																20,500
Potential milestone payments	5,000																5,000				30,500
Roche \| Undisclosed Target
Collaborative Agreements disclosures
Payments received under collaboration agreement																					5,000
Potential milestone payments																					$ 38,000
Number of undisclosed targets with exclusive licenses \| item																					4
License exercise fee																					$ 1,000
Term of agreement																					8 years
Roche \| Undisclosed Target \| Development milestones
Collaborative Agreements disclosures
Potential milestone payments																					$ 8,000
Roche \| Undisclosed Target \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments																					20,000
Roche \| Undisclosed Target \| Sales milestones
Collaborative Agreements disclosures
Potential milestone payments																					$ 10,000
Roche \| Undisclosed Target \| IND application filed
Collaborative Agreements disclosures
Potential milestone payments	$ 1,000																$ 1,000