Stock based compensation - 2014 Stock Option and Grant Plan - (Details)			1 Months Ended		12 Months Ended
	Jan. 01, 2017 shares	Jan. 01, 2016 shares	Oct. 31, 2015 shares	Jan. 31, 2014 $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) 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]
Shares available for future issuance (in shares)					5,394,249		4,723,719
Stock-based compensation | $					$ 6,310,000	$ 4,027,000	$ 425,000
Unrecognized stock-based compensation expense | $					$ 10,700,000
2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)					1,825,174
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
Restricted stock granted, original issued price (in dollars per share) | $ / shares					$ 0.04
2015 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares added to stock option plan	267,658	267,492
Number of shares authorized for stock-based compensation plan (in shares)			262,362
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
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
Employees restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 500,000	500,000	200,000
Employees restricted stock | 2014 Stock Option and Grant Plan - Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)					1,597,988
Non-employee consultants restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 2,600,000	2,600,000	$ 200,000
Non-employee consultants restricted stock | 2014 Stock Option and Grant Plan - Non-employee Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)					110,960
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 1,100,000
Restricted stock granted (in shares)				1,188,233
Restricted stock granted, original issued price (in dollars per share) | $ / shares				$ 0.0425
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
Stock options, all inclusive
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense | $					$ 8,700,000
Stock options granted (in shares)					989,560		0
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 3.0	700,000
Employees and directors stock options | 2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)					966,060
Vesting period					4 years
Employees and directors stock options | 2014 Stock Option and Grant Plan (2014 Plan) | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
Vesting percentage					25.00%
Employees and directors stock options | 2014 Stock Option and Grant Plan (2014 Plan) | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					3 years
Vesting percentage					75.00%
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $					$ 200,000	$ 300,000
Non-employees stock options | 2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)					23,500
Non-employees stock options | 2014 Stock Option and Grant Plan (2014 Plan) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
Non-employees stock options | 2014 Stock Option and Grant Plan (2014 Plan) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years