Represents the average involvement period over which the upfront payments on single-target licenses are amortized.

No definition available.

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

- Definition

Represents the period over which the upfront payments under the collaborative agreements are deferred.

+ References

No definition available.

+ Details

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

- Definition

Represents the difference between the entity's full cost to manufacture preclinical and clinical materials on behalf of its collaborators as compared to total amounts received from collaborators.

+ References

No definition available.

+ Details

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

- Definition

Represents the extended period after product launch that the company will earn royalty payments under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementsExtendedPeriodToEarnRoyaltyPayments
Namespace Prefix:	imgn_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period after product launch in which the company will earn royalty payments under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementsPeriodAfterProductLaunchToEarnRoyaltyPayments
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
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 number of licenses to multiple individual targets.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of licenses to two related targets.

+ References

No definition available.

+ Details

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

Represents the number of types of licensing and development agreements with collaborative partners.

+ References

No definition available.

+ Details

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

- Definition

Represents the types of milestone payments under collaborative arrangements.

+ References

No definition available.

+ Details

Name:	imgn_NumberOfTypesOfMilestonePaymentsUnderCollaborativeArrangements
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

Represents the number of quarters in arrear for revenue recognition.

+ References

No definition available.

+ Details

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

- Definition

Represents percentage of royalty.

+ References

No definition available.

+ Details

Name:	imgn_RoyaltyRevenuePercentage
Namespace Prefix:	imgn_
Data Type:	num: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:	num:percentItemType
Balance Type:	na
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 when royalties to the purchaser reach a certain milestone. 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

No definition available.

+ Details

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

- Definition

Represents information pertaining to transaction costs incurred during royalty purchase agreement.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Cash received for royalties 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),(c)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

+ Details

Name:	us-gaap_ProceedsFromRoyaltiesReceived
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=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ 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_AmgenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
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:	us-gaap_TypeOfArrangementAxis=imgn_RightToTestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Summary of Significant Accounting Policies (Details) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended			40 Months Ended	1 Months Ended				2 Months Ended	13 Months Ended	1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015 item	Mar. 31, 2014	Dec. 31, 2012	Nov. 30, 2009	Sep. 30, 2009	Dec. 31, 2012 item	May 31, 2013 item	Aug. 31, 2013	Dec. 31, 2011 item	Oct. 31, 2014 item	Nov. 30, 2013 item	Oct. 31, 2014 item	Oct. 31, 2010 item	Dec. 31, 2013	Apr. 30, 2015
Summary of Significant Accounting Policies
Royalties on net sales of Kadcyla	$ 5,099,000us-gaap_RoyaltyRevenue	$ 2,558,000us-gaap_RoyaltyRevenue	$ 13,890,000us-gaap_RoyaltyRevenue	$ 6,946,000us-gaap_RoyaltyRevenue
Number of types of licensing and development agreements with collaborative partners			2
License and milestone fees	5,078,000imgn_LicenseAndMilestoneFees	305,000imgn_LicenseAndMilestoneFees	52,729,000imgn_LicenseAndMilestoneFees	39,150,000imgn_LicenseAndMilestoneFees
Development and Commercialization License
Summary of Significant Accounting Policies
Average involvement period over which the upfront payments on single-target licenses are amortized			6 years 6 months
Difference between the full cost to manufacture and amounts received from collaborators for preclinical and clinical materials			8,700,000	1,600,000
Number of types of milestone payments under collaborative arrangements			3
Number of quarters in arrear for revenue recognition			1
Development and Commercialization License \| Minimum
Summary of Significant Accounting Policies
Period to earn royalty payments			10 years
Development and Commercialization License \| Maximum
Summary of Significant Accounting Policies
Period to earn royalty payments			12 years
Development and Commercialization License \| Kadcyla
Summary of Significant Accounting Policies
Extended period to earn royalty payments			2 years
Development and Commercialization License \| Kadcyla \| Minimum
Summary of Significant Accounting Policies
Period to earn royalty payments			10 years
Development and Commercialization License \| Kadcyla \| Maximum
Summary of Significant Accounting Policies
Period to earn royalty payments			12 years
Right-to-test agreement \| Minimum
Summary of Significant Accounting Policies
Average period over which upfront payments are deferred and recognized			12 months
Right-to-test agreement \| Maximum
Summary of Significant Accounting Policies
Average period over which upfront payments are deferred and recognized			18 months
Amgen \| Development and Commercialization License \| Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses			4					3
License exercise fee					1,000,000	1,000,000	1,000,000
License and milestone fees				2,200,000
Amgen \| Development and Commercialization License \| Non-exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses									1
License exercise fee									500,000
Bayer HealthCare \| Development and Commercialization License \| Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses			1
Biotest \| Development and Commercialization License \| Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses			1
Lilly \| Development and Commercialization License
Summary of Significant Accounting Policies
License and milestone fees			15,600,000							7,800,000
Lilly \| Development and Commercialization License \| Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses			3
Lilly \| Right-to-test agreement
Summary of Significant Accounting Policies
Number of single-target licenses											3
Novartis \| Development and Commercialization License
Summary of Significant Accounting Policies
Number of single-target licenses												3	2
License and milestone fees			25,700,000	17,200,000
Novartis \| Development and Commercialization License \| Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses			5									3		5
Number of licenses to two related targets			1
Number of related targets			2
Novartis \| Right-to-test agreement \| Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses															6
Roche \| Development and Commercialization License \| Kadcyla
Summary of Significant Accounting Policies
Royalties on net sales of Kadcyla	5,100,000	2,600,000	13,900,000	6,900,000
Roche \| Development and Commercialization License \| Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses			5
Sanofi \| Development and Commercialization License
Summary of Significant Accounting Policies
License exercise fee																2,000,000
License and milestone fees			1,600,000
Sanofi \| Development and Commercialization License \| Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses			1
Number of licenses to multiple individual targets			1
Subsequent event \| Kadcyla
Summary of Significant Accounting Policies
Percentage of royalty payments if applicable threshold is met																	85.00%
Subsequent event \| Immunity Royalty Holdings, L.P
Summary of Significant Accounting Policies
Cash proceeds from transaction																	200,000,000
Transaction costs incurred during royalty purchase agreement																	5,700,000
Subsequent event \| Immunity Royalty Holdings, L.P \| Kadcyla
Summary of Significant Accounting Policies
Percentage of royalty payments																	100.00%
Royalties on net sales of Kadcyla																	235,000,000
Royalties on net sales of Kadcyla when royalties to the purchaser reach a specified milestone																	260,000,000
Percentage of royalty payments if applicable threshold is met																	15.00%