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 milestone payments received.

+ References

No definition available.

+ Details

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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 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

- Details

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

- 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=31042434&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=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_RocheMember
Namespace Prefix:
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Period Type:

Collaborative Agreements (Details) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended	179 Months Ended
Collaborative Agreements (Details) (USD $)	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	May 31, 2000	Mar. 31, 2015
Collaborative Agreements disclosures
Amount of arrangement consideration included in license and milestone fees	$ 5,078,000imgn_LicenseAndMilestoneFees	$ 305,000imgn_LicenseAndMilestoneFees	$ 52,729,000imgn_LicenseAndMilestoneFees	$ 39,150,000imgn_LicenseAndMilestoneFees
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
Roche \| Development and Commercialization License \| Kadcyla
Collaborative Agreements disclosures
Payments received under collaboration agreement					2,000,000
Period in arrears to receive royalty reports and payments related to sales of Kadcyla			3 months
Royalties on net sales of Kadcyla	5,100,000	2,600,000	13,900,000	6,900,000
Roche \| Development and Commercialization License \| Kadcyla \| Maximum
Collaborative Agreements disclosures
Potential milestone payments					44,000,000
Roche \| Development and Commercialization License \| Kadcyla \| Development milestones
Collaborative Agreements disclosures
Payments received under collaboration agreement						13,500,000
Potential milestone payments					13,500,000
Roche \| Development and Commercialization License \| Kadcyla \| Regulatory milestones
Collaborative Agreements disclosures
Payments received under collaboration agreement						20,500,000
Potential milestone payments	5,000,000		5,000,000		30,500,000	5,000,000
Roche \| Development and Commercialization License \| Kadcyla \| Regulatory milestones in Japan
Collaborative Agreements disclosures
Payments received under collaboration agreement						5,000,000
Amount of arrangement consideration included in license and milestone fees			$ 10,000,000
Number of milestone payments received						2