Stock-based compensation (Details) - shares	1 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Stock based compensation
Shares reserved for future issuance (in shares)	2,814,716	9,445,451
2015 Plan
Stock based compensation
Shares available for grant (in shares)	1,068,287
Shares reserved for future issuance (in shares)	1,094,228
Shares available for future issuance (in shares)	1,038,758
2015 Plan | Annual minimum increase
Stock based compensation
Shares available for grant (in shares)	1,297,334
Percentage of outstanding Common Stock available for issuance (as a percent)	4.00%
2008 Plan
Stock based compensation
Shares available for grant (in shares)	25,942
Shares reserved for future issuance (in shares)	1,478,731
Shares available for future issuance (in shares)	0