Represents the initial period of the collaborative agreement.

No definition available.

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- Definition

Represents the estimated utilization period after commercialization of the product, which is used to allocate fees to the deliverables in the contract.

+ References

No definition available.

+ Details

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- Definition

Represents the amount of potential payments that could be received under the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

The number of hematology-related antibody-drug conjugate (ADC) programs under the collaborative agreement.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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- Definition

The number of early stage hematology-related antibody-drug conjugate (ADC) programs under the collaborative agreement.

+ References

No definition available.

+ Details

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Namespace Prefix:	imgn_
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- Definition

The number of product for which there is right to co-commercialize in the U.S. under the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

The number of product for which there is right to co-commercialize under certain limited circumstances in the U.S. under the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

The amount of reimbursement received under collaborative arrangement.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	imgn_
Data Type:	xbrli:integerItemType
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- 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

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- Definition

Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 13.A.3)
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 13.A.4(a).Q1)
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

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Balance Type:	credit
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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- 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=98514028&loc=d3e3536-108585

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Agreements - Jazz (Details) $ in Millions	1 Months Ended	12 Months Ended
Agreements - Jazz (Details) $ in Millions	Aug. 31, 2017 USD ($) product item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Agreements disclosures
Remaining arrangement consideration to be recognized as license revenue		$ 6.8	$ 7.1
Jazz
Collaborative Agreements disclosures
Number of early stage ADC programs \| item	2
Number of ADC programs \| item	3
Number of products with rights to co-commercialize \| product	1
Number of products with rights under certain limited circumstances \| product	2
Payments received under collaboration agreement	$ 75.0
Potential milestone payments	$ 100.0
Term of agreement	7 years
Offset to research and development expense		3.3
Number of development and commercialization licenses \| item	3
Discount rate (as a percent)	14.00%
Estimated utilization period after commercialization	10 years
Remaining arrangement consideration to be recognized as license revenue		$ 75.0