Stock based compensation - 2014 Stock Option and Grant Plan - (Details) $ / shares in Units, $ in Thousands				1 Months Ended		3 Months Ended		27 Months Ended
	Jan. 01, 2018 shares	Jan. 01, 2017 shares	Jan. 01, 2016 shares	Oct. 31, 2015 shares	Jan. 31, 2014 $ / shares shares	Mar. 31, 2018 USD ($) item shares	Mar. 31, 2017 USD ($) shares	Mar. 31, 2018 USD ($) item shares	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 | $						$ 2,322	$ 2,165
Stock options granted (in shares)						926,091
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)				262,362
Common stock shares authorized for issuance	321,300	267,658	267,492
Period to purchase shares						6 months
Increase in ESSP shares (as a percent)						1.00%
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		1,988,503		1,988,503
Percentage limit of issued and outstanding shares upto which additional shares may be issued						4.00
Number of shares authorized for stock-based compensation plan (in shares)	1,285,200	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 | $						$ 200		$ 200
Performance based milestone awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $						200		$ 1,500
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
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance milestones | item						1
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
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance milestones | item						3
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 | $						$ 1,900	1,300
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)						926,091
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $						$ 100	$ 200
Stock options granted (in shares)						0	0
Non-employees stock options | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued						0
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $						$ 1,198	$ 1,258
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $						$ 1,124	$ 907