Stock incentive plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Number of options
Options expired (in shares)	(50,000)
Weighted-average exercise price
Options expired (in dollars per share)	$ 4.00
Additional information
Closing price of common stock as reported on NASDAQ (in dollars per share)			$ 2.78	$ 3.05	$ 2.08
Stock options
Number of options
Outstanding at the beginning of the period (in shares)	7,066,876
Options granted (in shares)	1,869,446
Options exercised (in shares)	(946,475)
Options cancelled (in shares)	(505,347)
Options forfeited (in shares)	(95,204)
Outstanding at the end of the period (in shares)	7,339,296	7,066,876
Options exercisable at the end of the period (in shares)				5,315,067
Vested and expected to vest at the end of the period (in shares)				7,289,098
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 2.44
Options granted (in dollars per share)	2.50
Options exercised (in dollars per share)	$ 2.24
Options cancelled (in dollars per share)	2.70
Options forfeited (in dollars per share)	$ 2.97
Outstanding at the end of the period (in dollars per share)	$ 2.45	$ 2.44
Options exercisable (in dollars per share)				$ 2.39
Vested and expected to vest at the end of the period (in dollars per share)				$ 2.44
Weighted average remaining contractual life (years)
Outstanding at the end of the period	5 years 9 months 18 days	5 years 2 months 19 days
Exercisable at the end of the period	4 years 2 months 5 days
Vested and expected to vest at the end of the period	5 years 6 months
Aggregate intrinsic value
Outstanding at the end of the period				$ 4,587,530	$ 885,057
Exercisable at the end of the period				3,613,601
Vested and expected to vest at the end of the period				$ 4,566,241
Additional information
Weighted-average grant date fair value for options granted during the period (in dollars per share)	$ 0.92	$ 1.36	$ 1.14
Total fair value of options vested during the period	$ 784,965	$ 775,899	$ 949,588
Total intrinsic value of options exercised during the period	$ 1,090,711	$ 803,141	$ 212,580
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)	$ 3.64
Restricted stock
Stock-based expense
Number of shares outstanding	1,641,585	1,641,585		1,224,737	1,641,585
Number of Awards
Non-vested awards at the beginning of the period (in shares)	1,641,585
Awards granted (in shares)	752,835
Awards vested (in shares)	(158,467)
Awards cancelled and forfeited (in shares)	(1,011,216)
Non-vested awards at the end of the period (in shares)	1,224,737	1,641,585
Weighted Average Grant Date Fair Value
Non-vested awards at the beginning of the period (in dollars per share)	$ 1.13
Awards granted (in dollars per share)	1.54
Awards vested (in dollars per share)	2.68
Awards cancelled and forfeited (in dollars per share)	0.74
Non-vested awards at the end of the period (in dollars per share)	$ 1.71	$ 1.13
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
2000 Stock Incentive Plan
Stock-based expense
Shares available for future issuance				0
2000 Stock Incentive Plan | Stock options
Number of options
Outstanding at the beginning of the period (in shares)	50,000
Outstanding at the end of the period (in shares)	50,000	50,000
2006 Equity Incentive Plan
Stock-based expense
Shares available for future issuance				0	0
Aggregate shares authorized for issuance				7,621,127	8,245,761
Number of options issued below fair value (in shares)				0
2006 Equity Incentive Plan | Restricted stock
Number of Awards
Cumulative Equity Instruments	594,750
2012 Equity Incentive Plan
Stock-based expense
Shares available for future issuance				5,121,052
Aggregate shares authorized for issuance				8,874,274
Equity Incentive Plan 2012 And 2006 Member
Stock-based expense
Aggregate shares authorized for issuance				5,000,000