Stock Incentive Plans - Employee and non-employee options (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Stock incentive awards
Aggregate intrinsic value
Aggregate intrinsic value, vested and expected | $	$ 900	$ 300
Stock incentive awards | 2008 Plan
Number of options
Outstanding (period start) | shares	1,247,160
Granted | shares	489,177
Issuance of common stock upon exercise of options (in shares) | shares	(21,088)
Forfeited | shares	(3,462)
Outstanding (period end) | shares	1,711,787	1,247,160
Number of options, vested | shares	740,887
Number of options, vested and expected to vest | shares	1,565,466
Weighted-average exercise price
Balance at the beginning of the period (in dollars per share) | $ / shares	$ 5.05
Options granted (in dollars per share) | $ / shares	12.60
Options cancelled (in dollars per share) | $ / shares	6.74
Options exercised (in dollars per share) | $ / shares	2.16
Balance at the end of the period (in dollars per share) | $ / shares	$ 7.24	$ 5.05
Weighted-average exercise price, Vested (in dollars per share) | $ / shares	3.30
Weighted-average exercise price, vested and expected to vest | $ / shares	$ 6.89
Weighted-average remaining contractual year
Weighted-average remaining contractual term (in years)	7 years 9 months 15 days	7 years 4 months 28 days
Weighted-average remaining contractual term, Vested (in years)	5 years 7 months 17 days
Weighted-average remaining contractual term, vested and expected (in years)	7 years 7 months 13 days
Aggregate intrinsic value
Aggregate intrinsic value | $	$ 11,574	$ 3,130
Aggregate intrinsic value, vested | $	7,929
Aggregate intrinsic value, vested and expected | $	$ 11,134
Non-employee stock options | 2008 Plan
Number of options
Outstanding (period start) | shares	322,220
Granted | shares	84,615
Issuance of common stock upon exercise of options (in shares) | shares	(5,084)
Outstanding (period end) | shares	401,751	322,220
Number of options, vested | shares	320,339
Number of options, vested and expected to vest | shares	401,594
Weighted-average exercise price
Balance at the beginning of the period (in dollars per share) | $ / shares	$ 2.93
Options granted (in dollars per share) | $ / shares	7.12
Options exercised (in dollars per share) | $ / shares	0.47
Balance at the end of the period (in dollars per share) | $ / shares	$ 3.84	$ 2.93
Weighted-average exercise price, Vested (in dollars per share) | $ / shares	2.96
Weighted-average exercise price, vested and expected to vest | $ / shares	$ 3.84
Weighted-average remaining contractual year
Weighted-average remaining contractual term (in years)	6 years 7 days	5 years 6 months
Weighted-average remaining contractual term, Vested (in years)	5 years 1 month 17 days
Weighted-average remaining contractual term, vested and expected (in years)	6 years 7 days
Aggregate intrinsic value
Aggregate intrinsic value | $	$ 4,081	$ 1,288
Aggregate intrinsic value, vested | $	3,536
Aggregate intrinsic value, vested and expected | $	$ 4,080