Stock incentive plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Additional information
Closing stock price as reported on NASDAQ /AIM (in dollars per share)	$ 3.48	$ 2.27	$ 1.35
Stock options
Number of options
Outstanding at the beginning of the period (in shares)	7,776,948
Options granted (in shares)	1,550,000
Options exercised (in shares)	(488,513)
Options cancelled (in shares)	(708,961)
Options forfeited (in shares)	(77,001)
Outstanding at the end of the period (in shares)	8,052,473	7,776,948
Options exercisable at the end of the period (in shares)	6,363,090
Vested and expected to vest at the end of the period (in shares)	8,003,616
Weighted-average exercise price
Outstanding at the end of the period (in dollars per share)	$ 2.34	$ 2.27
Options exercisable at the end of the period (in dollars per share)	$ 2.30
Vested and expected to vest at the end of the period (in dollars per share)	$ 2.34
Weighted average remaining contractual life (years)
Outstanding at the end of the period	5 years 5 months 16 days	5 years 5 months 23 days
Exercisable at the end of the period	5 years 3 months 7 days
Vested and expected to vest at the end of the period	5 years 11 months 19 days
Aggregate intrinsic value
Outstanding at the end of the period	$ 9,221,448	$ 1,752,732
Exercisable at the end of the period	7,529,114
Vested and expected to vest at the end of the period	9,173,064
Additional information
Weighted-average grant date fair value for options granted during the period (in dollars per share)	$ 1.20	$ 0.70	$ 0.74
Total fair value of options vested during the period	1,126,087	549,802	537,199
Total intrinsic value of options exercised during the period	$ 360,371	$ 348,153	$ 12,583
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
2000 Stock Incentive Plan | Stock options
Number of options
Outstanding at the end of the period (in shares)	50,000	70,000
2006 Equity Incentive Plan
Stock incentive plan
Shares available for future issuance		2,325,866
Aggregate shares authorized for issuance	9,494,355	9,494,355
2006 Equity Incentive Plan | Restricted shares
Stock incentive plan
Shares issued	915,000	915,000
2012 Equity Incentive Plan
Stock incentive plan
Shares available for future issuance	6,656,328
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
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