Represents the denomination of the principal amount of debt that is used in conversion calculations.

No definition available.

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

- Definition

The number of trades that can be used to help determine the fair value of financial instruments.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

+ References

No definition available.

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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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Data Type:	xbrli:stringItemType
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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details)	6 Months Ended
	Jun. 30, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2017	Dec. 31, 2016 USD ($)
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%	4.50%
Number of trades \| item	0
Convertible Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%		4.50%
Principal amount of debt for conversion calculations				$ 1,000
Significant Other Observable Inputs (Level 2) \| Convertible Notes \| Face Value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value	$ 2,100,000
Significant Other Observable Inputs (Level 2) \| Convertible Notes \| Estimated fair value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value	3,000,000.0
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	204,807,000	$ 163,674,000
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	204,807,000	163,674,000
Recurring basis \| Significant Other Observable Inputs (Level 2)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	0	0
Recurring basis \| Significant Unobservable Inputs (Level 3)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 0	$ 0