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Shareholders' Equity (Details) (USD $)
In Millions, except Share data, unless otherwise specified
0 Months Ended
Aug. 18, 2014
Mar. 31, 2015
Dec. 31, 2014
Aug. 18, 2014
Shareholders' Equity        
Common stock, authorized shares   500,000,000us-gaap_CommonStockSharesAuthorized 500,000,000us-gaap_CommonStockSharesAuthorized  
Common stock, par value (in dollars per share)   $ 0.01us-gaap_CommonStockParOrStatedValuePerShare $ 0.01us-gaap_CommonStockParOrStatedValuePerShare  
Common stock, issued shares   50,001,770us-gaap_CommonStockSharesIssued 49,806,131us-gaap_CommonStockSharesIssued  
Common stock, outstanding shares   50,001,770us-gaap_CommonStockSharesOutstanding 49,806,131us-gaap_CommonStockSharesOutstanding  
Common stock disclosures        
Number of shares issued under underwritten public offering 10,235,000us-gaap_StockIssuedDuringPeriodSharesNewIssues      
Number of share's exercise in full of over-allotment option by underwriter 1,335,000insm_CommonStockSharesExerciseToUnderwritersInFullOfOverAllotmentOption     1,335,000insm_CommonStockSharesExerciseToUnderwritersInFullOfOverAllotmentOption
Shares price (in dollars per share) $ 11.25us-gaap_SharePrice     $ 11.25us-gaap_SharePrice
Net proceeds from the sale of shares $ 108.0us-gaap_ProceedsFromIssuanceOfCommonStock      
Underwriter's discount and offering expenses $ 7.1us-gaap_PaymentsOfStockIssuanceCosts      
Preferred stock        
Preferred stock, authorized   200,000,000us-gaap_PreferredStockSharesAuthorized    
Preferred stock, par value   $ 0.01us-gaap_PreferredStockParOrStatedValuePerShare    
Preferred stock, shares issued   0us-gaap_PreferredStockSharesIssued    
Preferred stock, shares outstanding   0us-gaap_PreferredStockSharesOutstanding    
Stock options        
Common stock disclosures        
Common stock shares reserved for issuance   4,821,435us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
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= insm_EmployeeAndDirectorsStockOptionsMember
   
Restricted stock units        
Common stock disclosures        
Common stock shares reserved for issuance   52,710us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
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= us-gaap_RestrictedStockUnitsRSUMember