Stock incentive plan (Tables)

12 Months Ended

Dec. 31, 2015

Stock-based compensation expense and assumptions

Summary of stock option activity

				Weighted-average
	Number of	Weighted-average		remaining contractual	Aggregate
	options	exercise price		life (in years)	intrinsic value
Outstanding as of December 31, 2014	7,066,876	$	2.44	5.22	$	885,057
Options granted	1,869,446		2.50
Options exercised	(946,475)		2.24
Options cancelled	(505,347)		2.70
Options expired	(50,000)		4.00
Options forfeited	(95,204)		2.97
Outstanding as of December 31, 2015	7,339,296		2.45	5.80		4,587,530
Options exercisable as of December 31, 2015	5,315,067		2.39	4.18		3,613,601
Vested and expected to vest as of December 31, 2015	7,289,098		2.44	5.50		4,566,241

Schedule of additional information pertaining to the stock options

	Year ended December 31,
	2015		2014		2013
Weighted-average grant date fair value for options granted during the period	$	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

Summary of the status of the non-vested shares of restricted stock grants

		Weighted-Average
	Number of	Grant Date Fair
	Awards	Value
Non-vested awards as of January 1, 2015	1,641,585	$	1.13
Awards granted	752,835		1.54
Awards vested	(158,467)		2.68
Awards repurchased	—		—
Awards cancelled and forfeited	(1,011,216)		0.74
Non-vested awards as of December 31, 2015	1,224,737	$	1.71