Stock-based compensation - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		6 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 23,513	$ 26,341
Shares withheld to satisfy exercise price of options (in shares)			153,727
Shares withheld to satisfy minimum tax withholding obligations (in shares)			107,765
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 9,100	12,300
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			14,400	14,000
Employee stock purchase plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 307	$ 359
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)	1,510,533
Shares available for future grants (in shares)			2,098,783
2015 Employee Stock Purchase Plan [Member] \| Employee stock purchase plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)		377,633
Common stock reserved for issuance (in shares)		1,481,992
Shares issued (in shares)			15,829

Spark Therapeutics, Inc. 10-Q