Stock incentive plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Number of options
Options expired (in shares)	(3,000)
Weighted-average exercise price
Options expired (in dollars per share)	$ 2.50
Additional information
Share price (in dollars per share)				$ 4.08	$ 3.05
Stock options
Number of options
Outstanding at the beginning of the period (in shares)	7,339,296
Options granted (in shares)	800,749
Options exercised (in shares)	(1,433,017)
Options cancelled (in shares)	(222,089)
Options forfeited (in shares)	(84,574)
Outstanding at the end of the period (in shares)	6,397,365	7,339,296
Options exercisable at the end of the period (in shares)				4,622,154
Vested and expected to vest at the end of the period (in shares)				6,354,908
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 2.45
Options granted (in dollars per share)	2.61
Options exercised (in dollars per share)	$ 2.45
Options cancelled (in dollars per share)	2.84
Options forfeited (in dollars per share)	$ 2.83
Outstanding at the end of the period (in dollars per share)	$ 2.45	$ 2.45
Options exercisable (in dollars per share)				$ 2.40
Vested and expected to vest at the end of the period (in dollars per share)				$ 2.45
Weighted average remaining contractual life (years)
Outstanding at the end of the period	5 years 5 months 23 days	5 years 9 months 18 days
Exercisable at the end of the period	4 years 8 months 23 days
Vested and expected to vest at the end of the period	5 years 9 months 7 days
Aggregate intrinsic value
Outstanding at the end of the period				$ 10,449,189	$ 4,587,530
Exercisable at the end of the period				7,784,568
Vested and expected to vest at the end of the period				$ 10,387,307
Additional information
Weighted-average grant date fair value for options granted during the period (in dollars per share)	$ 0.92	$ 0.92	$ 1.36
Total fair value of options vested during the period	$ 936,619	$ 784,965	$ 775,899
Total intrinsic value of options exercised during the period	$ 4,639,383	$ 1,090,711	$ 803,141
Exercise prices for options outstanding and exercisable, low end of the range (in dollars per share)	$ 1.20
Exercise prices for options outstanding and exercisable, high end of the range (in dollars per share)	$ 4.64
Restricted stock
Stock-based expense
Number of shares outstanding	1,224,737	1,224,737		606,596	1,224,737
Number of Awards
Non-vested awards at the beginning of the period (in shares)	1,224,737
Awards granted (in shares)	59,260
Awards vested (in shares)	(677,401)
Non-vested awards at the end of the period (in shares)	606,596	1,224,737
Weighted Average Grant Date Fair Value
Non-vested awards at the beginning of the period (in dollars per share)	$ 1.71
Awards granted (in dollars per share)	4.05
Awards vested (in dollars per share)	1.68
Non-vested awards at the end of the period (in dollars per share)	$ 1.97	$ 1.71
2012 Employee Stock Purchase Plan
Stock-based expense
Number of shares outstanding	0			0
Aggregate shares authorized for issuance				800,000
Increase in number of shares reserved for issuance (as a percent)	1.00%
Additional information
Specified percentage used to calculate the purchase price of shares of common stock	85.00%
Duration of each offering period	10 years
Number of Awards
Awards granted (in shares)	0
Non-vested awards at the end of the period (in shares)	0
2006 Equity Incentive Plan
Stock-based expense
Shares available for future issuance				0
Aggregate shares authorized for issuance				7,458,571	7,621,127
Number of options issued below fair value (in shares)				0
2006 Equity Incentive Plan | Restricted stock
Number of Awards
Cumulative Equity Instruments	396,500
2012 Equity Incentive Plan
Stock-based expense
Shares available for future issuance				5,431,947
Aggregate shares authorized for issuance				9,068,915
Equity Incentive Plan 2012 And 2006 Member
Stock-based expense
Aggregate shares authorized for issuance				5,000,000