Stock based compensation - 2014 Stock Option and Grant Plan - (Details) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 01, 2017 shares	Jan. 01, 2016 shares	Oct. 31, 2015 shares	Jan. 31, 2014 $ / shares shares	Sep. 30, 2017 USD ($) item shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) item shares	Sep. 30, 2016 USD ($)	Aug. 31, 2015 shares	Apr. 30, 2015 shares	Aug. 31, 2014 shares	Apr. 30, 2014 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 3,241	$ 1,789	$ 6,945	$ 4,528
Stock options granted (in shares)							1,541,000
2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for stock-based compensation plan (in shares)			2,998,823	823,529					2,669,411	2,047,058	2,000,000	1,411,764
2015 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	267,658		262,362		267,492		267,492
2015 Employee Stock Purchase Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payroll deduction percentage			1.00
2015 Employee Stock Purchase Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payroll deduction percentage			10.00
2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)			1,311,812		1,628,505		1,628,505
Percentage limit of issued and outstanding shares upto which additional shares may be issued							4.00
Additional shares added to stock option plan	1,070,635	1,069,971
Unvested restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense | $					$ 8,100		$ 8,100
Employees restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					100	100	400	400
Non-employee consultants restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					1,600	700	2,300	1,900
Performance based milestone awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 600	200	1,500	1,000
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 0
Restricted stock granted (in shares)				1,188,233
Restricted stock granted, original issued price (in dollars per share) | $ / shares				$ 0.0425
Number of awards | item					2		2
Founders restricted stock | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)				835,292
Founders restricted stock | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)				352,941
Founders restricted stock | Minimum | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				1 year
Founders restricted stock | Maximum | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 1,400	900	$ 3,800	2,200
Employees and directors stock options | 2015 Stock Option and Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)					265,500
Employees and directors stock options | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)							1,521,000
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 100	100	$ 300	100
Non-employees stock options | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued					20,000		20,000
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period					20,000		20,000
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 2,231	1,378	$ 4,107	3,425
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 1,010	$ 411	$ 2,838	$ 1,103