Fair value of financial instruments (Details 3) - Recurring basis - Level 3 - Derivative liability - Series C Warrants Issued with Series C Redeemable Preferred Stock Financing - Probability-Weighted Expected Return and Option Pricing - USD ($) $ / shares in Units, $ in Thousands	Jun. 30, 2015	Mar. 31, 2015	Feb. 28, 2015	Jan. 31, 2015	Dec. 31, 2014
Methodologies and significant inputs used in the determination of the fair value
Dividend yield (in dollars per share)	$ 0	$ 0	$ 0	$ 0	$ 0
Exercise price (in dollars per share)	$ 5	$ 5	$ 5	$ 5	$ 5
Volatility (as a percent)	60.00%	60.00%	60.00%	60.00%	60.00%
Number of warrants (in shares)	1,797,185	1,947,185	606,312	590,906	749,967
Fair value of liability at valuation date	$ 3,361	$ 3,233	$ 1,079	$ 1,052	$ 1,335
Minimum
Methodologies and significant inputs used in the determination of the fair value
Risk-free rate (as a percent)	0.14%	0.19%	0.25%	0.25%	0.25%
Contractual term	6 months	9 months	1 year	1 year	1 year
Maximum
Methodologies and significant inputs used in the determination of the fair value
Risk-free rate (as a percent)	2.83%	2.31%	2.47%	2.47%	2.47%
Contractual term	5 years	5 years	5 years	5 years	5 years