Stock incentive plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Number of options
Options expired (in shares)	(402,903)
Weighted-average exercise price
Options expired (in dollars per share)	$ 2.50
Additional information
Closing price of common stock as reported on NASDAQ (in dollars per share)	$ 2.08	$ 2.78	$ 3.48
Stock options
Number of options
Outstanding at the beginning of the period (in shares)	8,237,883
Options granted (in shares)	576,700
Options exercised (in shares)	(688,760)
Options cancelled (in shares)	321,520
Options forfeited (in shares)	(334,524)
Outstanding at the end of the period (in shares)	7,066,876	8,237,883
Options exercisable at the end of the period (in shares)	6,092,256
Vested and expected to vest at the end of the period (in shares)	7,028,232
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 2.38
Options granted (in dollars per share)	$ 3.18
Options exercised (in dollars per share)	$ 2.03
Options cancelled (in dollars per share)	2.59
Options forfeited (in dollars per share)	$ 2.85
Outstanding at the end of the period (in dollars per share)	$ 2.44	$ 2.38
Options exercisable (in dollars per share)	$ 2.36
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 2 months 19 days	5 years 7 months 13 days
Exercisable at the end of the period	4 years 7 months 24 days
Vested and expected to vest at the end of the period	5 years 2 months 12 days
Aggregate intrinsic value
Outstanding at the end of the period	$ 885,057	$ 3,766,674
Exercisable at the end of the period	884,337
Vested and expected to vest at the end of the period	885,021
Additional information
Weighted-average grant date fair value for options granted during the period (in dollars per share)	$ 1.36	$ 1.14	$ 1.20
Total fair value of options vested during the period	775,899	949,588	1,126,087
Total intrinsic value of options exercised during the period	$ 803,141	$ 212,580	$ 360,371
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)	$ 4.00
Restricted stock awards
Stock-based expense
Shares issued	930,830
Number of shares outstanding	1,641,585
Number of Awards
Non-vested awards at the beginning of the period (in shares)	1,765,333
Awards granted (in shares)	930,830
Awards vested (in shares)	(132,902)
Awards repurchased (in dollars)	(320,250)
Awards cancelled and forfeited (in shares)	(601,426)
Non-vested awards at the end of the period (in shares)	1,641,585
Weighted Average Grant Date Fair Value
Non-vested awards at the beginning of the period (in dollars per share)	$ 2.08
Awards granted (in dollars per share)	$ 0.60
Awards vested (in dollars per share)	$ 2.79
Awards repurchased (in dollars per share)	1.97
Awards cancelled and forfeited (in dollars per share)	$ 2.29
Non-vested awards at the end of the period (in dollars per share)	$ 1.13
2000 Stock Incentive Plan
Stock-based expense
Shares available for future issuance	0	0
2000 Stock Incentive Plan | Stock options
Number of options
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	8,245,761	9,330,588
Number of options issued below fair value (in shares)	0	0
2006 Equity Incentive Plan | Restricted stock awards
Stock-based expense
Shares issued	594,750	594,750
Number of Awards
Awards granted (in shares)	594,750	594,750
2012 Equity Incentive Plan
Stock-based expense
Shares available for future issuance	8,240,800
Aggregate shares authorized for issuance	5,000,000
Increase in number of shares reserved for issuance (as a percent)	4.00%
Employee Stock
Stock-based expense
Shares issued	0
Number of shares outstanding	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