Stock incentive plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Additional information
Closing price of common stock as reported on NASDAQ (in dollars per share)	$ 2.78	$ 3.48	$ 2.27
Stock options
Number of options
Outstanding at the beginning of the period (in shares)	8,052,473
Options granted (in shares)	949,300
Options exercised (in shares)	(540,879)
Options cancelled (in shares)	(65,666)
Options forfeited (in shares)	(157,345)
Outstanding at the end of the period (in shares)	8,237,883	8,052,473
Options exercisable at the end of the period (in shares)	6,774,584
Vested and expected to vest at the end of the period (in shares)	8,190,603
Weighted-average exercise price
Outstanding at the end of the period (in dollars per share)	$ 2.38	$ 2.34
Options exercisable at the end of the period (in dollars per share)	$ 2.31
Vested and expected to vest at the end of the period (in dollars per share)	$ 2.38
Weighted average remaining contractual life (years)
Outstanding at the end of the period	5 years 7 months 13 days	5 years 5 months 16 days
Exercisable at the end of the period	4 years 11 months 1 day
Vested and expected to vest at the end of the period	5 years 7 months 6 days
Aggregate intrinsic value
Outstanding at the end of the period	$ 3,766,674	$ 9,221,448
Exercisable at the end of the period	3,565,363
Vested and expected to vest at the end of the period	3,761,252
Additional information
Weighted-average grant date fair value for options granted during the period (in dollars per share)	$ 1.14	$ 1.20	$ 0.70
Total fair value of options vested during the period	949,588	1,126,087	549,802
Total intrinsic value of options exercised during the period	$ 212,580	$ 360,371	$ 348,153
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 incentive plan
Shares issued	875,583
Number of shares outstanding	1,765,333
Number of Awards
Non-vested awards at the beginning of the period (in shares)	915,000
Awards granted (in shares)	875,583
Awards vested (in shares)	(16,250)
Awards cancelled and forfeited (in shares)	(9,000)
Non-vested awards at the end of the period (in shares)	1,765,333
Weighted Average Grant Date Fair Value
Non-vested awards at the beginning of the period (in dollars per share)	$ 1.97
Awards granted (in dollars per share)	$ 2.22
Awards vested (in dollars per share)	$ 2.92
Awards cancelled and forfeited (in dollars per share)	$ 2.92
Non-vested awards at the end of the period (in dollars per share)	$ 2.08
2000 Stock Incentive Plan
Stock incentive plan
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 incentive plan
Shares available for future issuance	0	0
Aggregate shares authorized for issuance	9,330,588	9,494,355
Number of options issued below fair value (in shares)	0	0
2006 Equity Incentive Plan | Restricted stock awards
Stock incentive plan
Shares issued	915,000	915,000
Number of Awards
Awards granted (in shares)	915,000	915,000
2012 Equity Incentive Plan
Stock incentive plan
Shares available for future issuance	7,037,950
Aggregate shares authorized for issuance	5,000,000
Increase in number of shares reserved for issuance (as a percent)	4.00%
2012 Employee Stock Purchase Plan
Stock incentive plan
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