Stock based compensation (Details)					1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2019 shares	Jan. 01, 2018 shares	Jan. 01, 2017 shares	Jan. 01, 2016 shares	Oct. 31, 2015 shares	Jan. 31, 2014 $ / shares shares	Sep. 30, 2019 USD ($) item $ / shares shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2019 USD ($) item $ / shares shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2016 item	Dec. 31, 2018 $ / 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 | $							$ 3,349,000	$ 2,636,000	$ 12,120,000	$ 13,072,000
Stock options granted (in shares)									1,538,515
Exercise price | $ / shares							$ 16.04		$ 16.04			$ 15.91
Granted (in dollars per share) | $ / shares									$ 15.50
2014 Stock Option and Grant 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	325,707	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		2,002,018		2,002,018
Percentage limit of issued and outstanding shares up to which additional shares may be issued									4.00%
Common stock shares authorized for issuance	1,302,830	1,285,200	1,070,635	1,069,971
Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted or RSUs issued outside of Stock Option Plan							0
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 3,008,000	2,559,000	$ 10,312,000	12,428,000
Stock options | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued									58,125
Vesting period									4 years
Stock options | Inducement Awards | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage									25.00%
Stock options | Inducement Awards | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									36 months
Vesting percentage									75.00%
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							259,000	$ 24,000	$ 1,609,000	$ 480,000
Unrecognized stock-based compensation expense | $							$ 3,700,000		$ 3,700,000
Vesting period							3 years		3 years
Granted (in shares)								0	445,337	0
Restricted stock granted, original issued price (in dollars per share) | $ / shares							$ 25.40		$ 11.65
Restricted stock units | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)							25,880		387,212
Restricted stock units | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									3 years
Restricted stock units | Inducement Awards | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage									33.00%
Restricted stock units | Inducement Awards | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage									33.00%
Restricted stock units | Inducement Awards | Vesting period, tranche three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage									33.00%
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 | $								$ 100,000	$ 0	$ 300,000
Compensation expense for remaining founders' awards | $									$ 0
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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
Non-statutory stock options | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued							338,750		338,750
Share-based Payment Arrangement, Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 3,000,000.0	2,500,000	$ 10,400,000	12,300,000
Share-based Payment Arrangement, Employee | 2014 Stock Option and Grant Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									4 years
Share-based Payment Arrangement, Employee | 2014 Stock Option and Grant Plan | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									1 year
Vesting percentage									25.00%
Share-based Payment Arrangement, Employee | 2014 Stock Option and Grant Plan | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									3 years
Vesting percentage									75.00%
Share-based Payment Arrangement, Employee | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued							227,595		1,199,765
Share-based Payment Arrangement, Employee | Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 300		$ 1,600,000
Share-based Payment Arrangement, Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)							0		0
Share-based Payment Arrangement, Nonemployee | 2014 Stock Option and Grant Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									1 year
Share-based Payment Arrangement, Nonemployee | 2014 Stock Option and Grant Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									4 years
Share-based Payment Arrangement, Nonemployee | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued							0		0
Share-based Payment Arrangement, Nonemployee | Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 100,000	100,000	$ 100,000	300,000
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							1,304,000	1,176,000	5,975,000	3,706,000
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 2,045,000	$ 1,460,000	$ 6,145,000	$ 9,366,000