Stock based compensation (Details) $ / shares in Units, $ in Thousands					1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 16, 2018 $ / shares shares	Jan. 01, 2018 shares	Jan. 01, 2017 shares	Jan. 01, 2016 shares	Oct. 31, 2015 shares	Jan. 31, 2014 $ / shares shares	Sep. 30, 2018 USD ($) item $ / shares shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2018 USD ($) item $ / shares shares	Sep. 30, 2017 USD ($) shares	Dec. 31, 2016 item	Dec. 31, 2017 $ / 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 | $							$ 2,636	$ 3,241	$ 13,072	$ 6,945
Stock options granted (in shares)									2,030,191
Exercise price | $ / shares							$ 16.02		$ 16.02			$ 11.82
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]
Common stock shares authorized for issuance		321,300	267,658	267,492
Number of shares authorized for stock-based compensation plan (in shares)					262,362
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		2,055,980		2,055,980
Percentage limit of issued and outstanding shares up to which additional shares may be issued									4.00%
Common stock shares authorized for issuance		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 | $							$ 100		$ 100
Performance based milestone awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Probable performance milestones | item											1
Performance milestones | item											3
Stock-based compensation | $							$ 100	300	300	900
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 | $							$ 2,500	1,400	$ 12,300	3,800
Employees and directors stock options | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued							846,000		2,030,191
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 100	$ 100	$ 300	$ 300
Stock options granted (in shares)							0	20,000	0	20,000
Non-employees stock options | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued							0		0
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 1,176	$ 2,231	$ 3,706	$ 4,107
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 1,460	$ 1,010	$ 9,366	$ 2,838
Chief Executive Officer | Non-statutory stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for stock-based compensation plan (in shares)	650,000
Exercise price | $ / shares	$ 18.03
Vesting period	4 years
Chief Executive Officer | Non-statutory stock options | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	12 months
Options vested (as a percent)	25.00%
Chief Executive Officer | Non-statutory stock options | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	36 months
Options vested (as a percent)	75.00%