Represents the number of licenses terminated.

No definition available.

Name:	imgn_NumberOfLicensesTerminated
Namespace Prefix:	imgn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of single-target licenses the company has right to use.

+ References

No definition available.

+ Details

Name:	imgn_NumberOfSingleTargetLicenses
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

Amount, before allowance for credit loss, of right to consideration in exchange for good or service transferred to customer, when right is conditioned on something other than passage of time.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130533-203044

+ Details

Name:	us-gaap_ContractWithCustomerAssetGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(a),(d))
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

+ Details

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

- Definition

Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

+ Details

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

- Details

Name:	srt_CounterpartyNameAxis=imgn_AmgenMember
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_RegulatoryMilestoneInvestigationalNewDrugApplicationFilingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_ProductOrServiceAxis=imgn_UpfrontPaymentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=imgn_LicenseAndMilestoneFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=imgn_ResearchAndDevelopmentSupportMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Agreements - Amgen/Oxford BioTherapeutics (Details)	1 Months Ended						3 Months Ended														6 Months Ended		12 Months Ended
Agreements - Amgen/Oxford BioTherapeutics (Details)	Dec. 31, 2018 USD ($)	Aug. 31, 2018 item	Feb. 29, 2016 item	Dec. 31, 2015 item	Oct. 31, 2013 USD ($)	May 31, 2013 USD ($) item	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2000 USD ($) item
Collaborative Agreements disclosures
Revenue from contract with customer							$ 13,416,000	$ 10,928,000	$ 9,287,000	$ 19,815,000	$ 39,448,000	$ 8,480,000	$ 39,020,000	$ 28,499,000	$ 13,846,000	$ 7,660,000	$ 7,408,000	$ 19,714,000	$ 18,029,000	$ 14,851,000	$ 21,506,000	$ 32,880,000	$ 53,446,000	$ 115,447,000	$ 48,628,000	$ 60,002,000
Amgen/Oxford BioTherapeutics
Collaborative Agreements disclosures
Revenue from contract with customer																							4,000,000
Number of single-target licenses \| item				4		1																					3
Number of licenses terminated \| item		1	2
Amgen/Oxford BioTherapeutics \| Development milestones
Collaborative Agreements disclosures
Potential milestone payment					$ 9,000,000
Amgen/Oxford BioTherapeutics \| IND application filed
Collaborative Agreements disclosures
Revenue from contract with customer	$ 1,000,000
Amgen/Oxford BioTherapeutics \| Phase 2 clinical trial
Collaborative Agreements disclosures
Potential milestone payment	$ 3,000,000						3,000,000																3,000,000
Amgen/Oxford BioTherapeutics \| Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment					20,000,000
Amgen/Oxford BioTherapeutics \| Milestone payments
Collaborative Agreements disclosures
Potential milestone payment					5,000,000
Upfront payment \| Amgen/Oxford BioTherapeutics
Collaborative Agreements disclosures
Revenue from contract with customer																											$ 1,000,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer							1,747,000	672,000	1,321,000	11,540,000	29,580,000	79,000	31,080,000	18,730,000	5,076,000	76,000	76,000	10,077,000	10,692,000	6,070,000	5,152,000	16,762,000	15,280,000	79,469,000	15,305,000	26,915,000
License and milestone fees \| Amgen/Oxford BioTherapeutics
Collaborative Agreements disclosures
Revenue from contract with customer					500,000	$ 500,000																	84,000
Potential milestone payment					$ 34,000,000
License and milestone fees \| Amgen/Oxford BioTherapeutics \| IND application filed
Collaborative Agreements disclosures
Revenue from contract with customer																										1,000,000
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer							$ 218,000	$ 388,000	$ 388,000	$ 383,000	$ 452,000	$ 650,000	$ 902,000	$ 1,478,000	$ 1,427,000	$ 1,354,000	$ 1,335,000	$ 1,059,000	$ 848,000	$ 772,000	2,781,000	$ 1,620,000	1,377,000	3,482,000	$ 5,175,000	4,014,000
Costs of services and materials																					$ 22,000
Research and development support \| Amgen/Oxford BioTherapeutics
Collaborative Agreements disclosures
Costs of services and materials																							$ 0	$ 0		$ 15,000