A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	imgn_ActivityWithinLiabilityAccountRollForward
Namespace Prefix:	imgn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate amount of royalties to be received before the company will receive proceeds for royalties.

+ References

No definition available.

+ Details

Name:	imgn_AggregateRoyaltiesThreshold
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:	imgn_ContractWithCustomerLiabilityLineItems
Namespace Prefix:	imgn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents effective annual interest rate for royalties.

+ References

No definition available.

+ Details

Name:	imgn_EffectiveAnnualInterestRate
Namespace Prefix:	imgn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the prospective estimated annual interest rate.

+ References

No definition available.

+ Details

Name:	imgn_EstimatedEffectiveAnnualInterestRate
Namespace Prefix:	imgn_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- 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 period in arrears to receive royalty reports and payments related to sales of Kadcyla.

+ References

No definition available.

+ Details

Name:	imgn_PeriodInArrearsToReceiveRoyaltyReportsAndPayments
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents proceeds received from the sale of residual rights to receive royalty payments.

+ References

No definition available.

+ Details

Name:	imgn_ProceedsReceivedFromSaleOfResidualRightsNet
Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents contingent broker fees.

+ References

No definition available.

+ Details

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

- Definition

Represents percentage of royalty.

+ References

No definition available.

+ Details

Name:	imgn_RoyaltyRevenuePercentage
Namespace Prefix:	imgn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents percentage of royalty if applicable threshold is met.

+ References

No definition available.

+ Details

Name:	imgn_RoyaltyRevenuePercentageIfApplicableThresholdIsMet
Namespace Prefix:	imgn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The transaction costs that are amortized to interest expense over the estimated life of the royalty purchase agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents transaction costs to conclude royalty agreements.

+ References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date of obligations incurred through that date and payable for royalties.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15(a))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15(5))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

+ Details

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

- Definition

The increase (decrease) during the reporting period in the obligations due for compensation payments related to the use of copyrights, patents, trade names, licenses, technology. Royalty payments are also paid by the lease holders for oil, gas, and mineral extraction.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

+ Details

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

- Definition

Cash received for royalties during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (a),(c)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3536-108585

+ Details

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

- Details

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

- Details

Name:	srt_CounterpartyNameAxis=imgn_ImmunityRoyaltyHoldingsL.pMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=imgn_OmersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=imgn_RocheMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Liability Related to Sale of Future Royalties (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended		12 Months Ended
	Jan. 31, 2019	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2015
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties		$ 9,027	$ 35,035
KADCYLA
Liability Related to Sale of Future Royalties
Percentage of royalty payments if applicable threshold is met				85.00%
IRH \| KADCYLA
Liability Related to Sale of Future Royalties
Percentage of royalty payments				100.00%
Percentage of royalty payments if applicable threshold is met				15.00%
Transaction costs for royalty agreements				$ 5,900
Change in liability related to sale of future royalties
Liability related to sale of future royalties, net - beginning balance		41,044
Proceeds from sale of future royalties, net		200,000		200,000
Royalty payments received and paid		(9,690)
Non-cash interest expense recognized		3,194
Liability related to sale of future royalties, net - ending balance		$ 34,548
Effective annual interest rate		10.50%
Current effective interest rate		8.7
IRH \| KADCYLA \| Maximum
Liability Related to Sale of Future Royalties
Royalties threshold				260,000
IRH \| KADCYLA \| Minimum
Liability Related to Sale of Future Royalties
Royalties threshold				$ 235,000
OMERS \| KADCYLA
Liability Related to Sale of Future Royalties
Percentage of royalty payments	100.00%
Non-cash royalty revenue related to the sale of future royalties	$ 65,200		3,700
Contingent broker fees	1,500
Net proceeds from sale of residual rights to receive royalty payments	$ 65,200	$ 12,500
Roche
Liability Related to Sale of Future Royalties
Period in arrears to receive royalty reports and payments related to sales of Kadcyla		3 months
Roche \| KADCYLA
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties		$ 21,500	$ 38,800