Stockholders' equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended		1 Months Ended	12 Months Ended											5 Months Ended	12 Months Ended			5 Months Ended	12 Months Ended			5 Months Ended	12 Months Ended			5 Months Ended	12 Months Ended
	Jun. 28, 2011	Jan. 11, 2011	Jul. 31, 2011	Dec. 31, 2011	Dec. 31, 2002	Dec. 31, 2012	May 03, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 28, 2011 Common stock warrants	Dec. 31, 2012 2001 Plan	Dec. 31, 2011 2001 Plan	Dec. 31, 2012 2011 Plan	Dec. 31, 2011 2011 Plan	May 03, 2011 Series A convertible preferred stock	Dec. 31, 2010 Series A convertible preferred stock	Dec. 31, 2009 Series A convertible preferred stock	Jul. 31, 2001 Series A convertible preferred stock	May 03, 2011 Series A-2 convertible preferred stock	Dec. 31, 2010 Series A-2 convertible preferred stock	Dec. 31, 2009 Series A-2 convertible preferred stock	Feb. 28, 2002 Series A-2 convertible preferred stock	May 03, 2011 Series B convertible preferred stock	Dec. 31, 2010 Series B convertible preferred stock	Dec. 31, 2009 Series B convertible preferred stock	Dec. 31, 2003 Series B convertible preferred stock	May 03, 2011 Series C convertible preferred stock	Dec. 31, 2010 Series C convertible preferred stock	Dec. 31, 2009 Series C convertible preferred stock	Sep. 30, 2006 Series C convertible preferred stock
Stockholders' equity (deficit)
Convertible preferred stock, shares issued																		5,052,566				1,104,969				3,433,326				10,983,188
Preferred stock issued (in shares)				0		0
Preferred stock outstanding (in shares)				0		0
Conversion price (in dollars per share)																		$ 3.00				$ 4.026072935				$ 3.00				$ 4.95835
Convertible preferred stock conversion ratio																		1				1.07				1				1.2
Multiplier for number of shares converted (in dollars per share)																						$ 4.30								$ 5.95
Cumulative dividends per annum on preferred stock (in dollars per share)															$ 0.15	$ 0.15	$ 0.15		$ 0.215	$ 0.215	$ 0.215		$ 0.15	$ 0.15	$ 0.15		$ 0.2975	$ 0.2975	$ 0.2975
Common stock authorized (in shares)				100,000,000		100,000,000
Common Stock reserved
Outstanding stock options (in shares)				6,601,000		5,045,000		5,288,000	5,281,000		2,345,000	4,340,000	2,700,000	2,261,000
Number of shares available for grant (in shares)													2,811,000	1,739,000
Total (in shares)				8,340,000		7,856,000
Note receivable from stockholder
Restricted common stock (in shares)					290,000
Restricted common stock at deemed fair value (in dollars per share)					$ 0.30
Cash proceeds from restricted common stock granted					$ 9
Note receivable from shareholder with an interest rate equal to the applicable federal rate					78
Forgiveness of note receivable from stockholder and principal and interest outstanding		103		103
Treasury stock
Retirement of outstanding shares of treasury stock			1,256,608
Retirement of treasury stock			$ 4,575
Common stock warrants
Common stock warrants outstanding (in shares)							25,196			26,322
Warrants exercised into shares (in shares)	21,525