Represents the number of common share equivalents under the treasury stock method as of the balance sheet date.

No definition available.

Name:	imgn_NumberCommonStockEquivalentShares
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Data Type:	xbrli:sharesItemType
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Period Type:	duration

- Definition

Represents the number of shares reserved for issuance under the equity-based awards agreement awarded under the plan that validly exist and are outstanding as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	imgn_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardsOutstandingNumber
Namespace Prefix:	imgn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Definition

Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainLossOnSaleOfPropertyPlantEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_PropertyPlantAndEquipmentLineItems
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Period Type:	duration

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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Summary of Significant Accounting Policies (Details 4) (USD $) Share data in Thousands, unless otherwise specified	12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2012
Property and Equipment
(Losses) gain on the sale/disposal of furniture and equipment (in dollars)	$ (20,000)	$ 21,000	$ (51,000)
Computation of Net Income (Loss) per Common Share
Options outstanding to purchase common stock and unvested restricted stock (in shares)	8,486	7,703	6,442
Common stock equivalents under treasury stock method (in shares)	1,820	2,149	2,194
Machinery and equipment
Property and Equipment
Estimated useful lives	5 years
Computer hardware and software
Property and Equipment
Estimated useful lives	3 years
Furniture and fixtures
Property and Equipment
Estimated useful lives	5 years
(Losses) gain on the sale/disposal of furniture and equipment (in dollars)	$ (20,000)	$ 21,000	$ (51,000)
Leasehold improvements \| Maximum
Property and Equipment
Estimated useful lives	7 years