Document and entity information - shares
6 Months Ended
Jun. 30, 2018
Aug. 01, 2018
Document And Entity Information [Abstract]    
Entity Registrant Name Spark Therapeutics, Inc.  
Entity Central Index Key 0001609351  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Document Type 10-Q  
Trading Symbol ONCE  
Document Period End Date Jun. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus (Q1,Q2,Q3,FY) Q2  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   37,571,535