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Capital Stock (Details) (USD $)
3 Months Ended 9 Months Ended 129 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Jun. 30, 2014
Jun. 30, 2013
2001 Director Plan | Stock options              
Stock-Based Compensation              
Stock compensation expense $ 18,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_DirectorPlan2001Member
$ 2,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_DirectorPlan2001Member
$ (19,000)us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_DirectorPlan2001Member
$ (11,000)us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_DirectorPlan2001Member
     
Stock units issued (in shares)         0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_DirectorPlan2001Member
   
Compensation Policy for Non-Employee Directors | Stock options              
Stock-Based Compensation              
Vesting period     1 year        
Stock options granted to directors (in shares)     80,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_CompensationPolicyNonEmployeeDirectorMember
    80,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_CompensationPolicyNonEmployeeDirectorMember
41,805us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_CompensationPolicyNonEmployeeDirectorMember
Compensation Policy for Non-Employee Directors | Deferred share units              
Stock-Based Compensation              
Stock compensation expense $ 93,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= imgn_DeferredShareUnitsMember
/ us-gaap_PlanNameAxis
= imgn_CompensationPolicyNonEmployeeDirectorMember
$ 118,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= imgn_DeferredShareUnitsMember
/ us-gaap_PlanNameAxis
= imgn_CompensationPolicyNonEmployeeDirectorMember
$ 329,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= imgn_DeferredShareUnitsMember
/ us-gaap_PlanNameAxis
= imgn_CompensationPolicyNonEmployeeDirectorMember
$ 315,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= imgn_DeferredShareUnitsMember
/ us-gaap_PlanNameAxis
= imgn_CompensationPolicyNonEmployeeDirectorMember
     
Vesting period     1 year