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Summary of Significant Accounting Policies (Details 4) (USD $)
0 Months Ended 3 Months Ended 9 Months Ended
Nov. 11, 2014
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
item
Mar. 31, 2014
Nov. 11, 2014
Additional disclosure for options            
Cash received for exercise of stock options       $ 1,432,000us-gaap_ProceedsFromStockOptionsExercised $ 8,557,000us-gaap_ProceedsFromStockOptionsExercised  
Financial Instruments and Concentration of Credit Risk            
Number of financial institutions in the U.S. in which cash and cash equivalents are primarily maintained       3imgn_CashAndCashEquivalentsNumberOfFinancialInstitutions    
2006 Plan            
Stock-Based Compensation            
Number of employee share-based compensation plans       1imgn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfPlans
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
   
Additional number of shares authorized for issuance 5,500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
         
Common stock authorized for issuance (in shares) 17,500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
        17,500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
Number of shares of common stock added to the Plan from the Former Plan       1,676,599imgn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAddedToPlanFromFormerPlan
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
   
Additional disclosure for options            
Stock compensation expense   3,600,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
3,700,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
12,500,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
12,100,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
 
Estimated fair value of unvested employee awards, net of estimated forfeitures       22,300,000imgn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedSharesTotalFairValue
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
   
Weighted average vesting period of unvested employee awards       2 years    
2006 Plan | Stock options            
Weighted-average assumptions used to estimate the fair value of each stock option            
Number of group of awards for which expected term is calculated for and applied       1imgn_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsNumberOfGroupForWhichExpectedTermCalculatedAndApplied
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
   
Dividend (as a percent)   0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
 
Volatility (as a percent)   60.76%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
60.44%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
60.46%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
60.44%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
 
Risk-free interest rate (as a percent)   1.67%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
1.94%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
1.86%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
1.74%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
 
Expected life   6 years 3 months 18 days 6 years 3 months 18 days 6 years 3 months 18 days 6 years 3 months 18 days  
Weighted-average grant date fair values of options granted (in dollars per share)   $ 3.83us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
$ 9.52us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
$ 6.06us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
$ 10.54us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
 
Additional disclosure for options            
Stock options exercised (in shares)       205,000us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
   
Cash received for exercise of stock options       $ 1,400,000us-gaap_ProceedsFromStockOptionsExercised
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
   
2006 Plan | Stock options | Maximum            
Stock-Based Compensation            
Vesting period       4 years    
Exercise period       10 years    
Additional disclosure for options            
Exercised (in dollars per share)       $ 9.88us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
2006 Plan | Stock options | Minimum            
Additional disclosure for options            
Exercised (in dollars per share)       $ 3.19us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= imgn_EmployeeDirectorsAndConsultantStockOptionsMember
/ us-gaap_PlanNameAxis
= imgn_StockIncentivePlan2006Member
/ us-gaap_RangeAxis
= us-gaap_MinimumMember