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 potential fee receivable for each license exercised under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementExerciseFeePotentialPerLicense
Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

Represents the number of compounds under development and commercialization licenses under the collaborative agreements entered into by the entity.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of milestone payments received.

+ References

No definition available.

+ Details

Name:	imgn_CollaborativeArrangementNumberOfMilestonePaymentsPaid
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 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

- 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 Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 27

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: 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=6943989&loc=d3e3536-108585

+ Details

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

Agreements (Details 2) (Amgen, USD $)	1 Months Ended	12 Months Ended			1 Months Ended				12 Months Ended						1 Months Ended	12 Months Ended		1 Months Ended
Agreements (Details 2) (Amgen, USD $)	Sep. 30, 2000 Right-to-test agreement Undisclosed Target	Jun. 30, 2013 Exclusive development and commercialization license item	Jun. 30, 2013 Exclusive development and commercialization license Development milestones	Jun. 30, 2013 Exclusive development and commercialization license Development milestones Phase II clinical trial	May 31, 2013 Exclusive development and commercialization license Undisclosed Target	Dec. 31, 2012 Exclusive development and commercialization license Undisclosed Target	Nov. 30, 2009 Exclusive development and commercialization license Undisclosed Target	Sep. 30, 2009 Exclusive development and commercialization license Undisclosed Target	Jun. 30, 2013 Exclusive development and commercialization license Undisclosed Target item	Sep. 30, 2000 Exclusive development and commercialization license Undisclosed Target	Sep. 30, 2000 Exclusive development and commercialization license Undisclosed Target Maximum	Sep. 30, 2000 Exclusive development and commercialization license Undisclosed Target Development milestones	Sep. 30, 2000 Exclusive development and commercialization license Undisclosed Target Regulatory milestones	Sep. 30, 2000 Exclusive development and commercialization license Undisclosed Target Sales milestones	Nov. 30, 2011 Exclusive development and commercialization license Undisclosed Target IND application effective item	Jun. 30, 2013 Non-exclusive development and commercialization license item	Jun. 30, 2013 Non-exclusive development and commercialization license Development milestones	May 31, 2013 Non-exclusive development and commercialization license Undisclosed Target item	Sep. 30, 2000 Non-exclusive development and commercialization license Undisclosed Target	Sep. 30, 2000 Non-exclusive development and commercialization license Undisclosed Target Maximum	Sep. 30, 2000 Non-exclusive development and commercialization license Undisclosed Target Development milestones	Sep. 30, 2000 Non-exclusive development and commercialization license Undisclosed Target Regulatory milestones	Sep. 30, 2000 Non-exclusive development and commercialization license Undisclosed Target Sales milestones
Collaborative Agreements disclosures
Term of agreement	10 years
Payments received under collaboration agreement	$ 5,000,000
License exercise fee, per license										1,000,000									500,000
Potential milestone payments receivable			1,000,000	3,000,000							34,000,000	9,000,000	20,000,000	5,000,000			500,000			17,000,000	4,500,000	10,000,000	2,500,000
Number of single-target licenses		3							3							1		1
Fee received per license					$ 500,000	$ 1,000,000	$ 1,000,000	$ 1,000,000							$ 1,000,000
Number of compounds under development and commercialization licenses															2
Number of milestone payments															2