| Net loss per share (Details) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
| Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013 | |
| Antidilutive securities excluded from the computation of diluted net loss per share | ||||
| Antidilutive securities excluded from computation of earnings per share (in shares) | 17,639 | 15,725 | 17,639 | 15,725 | 
| Employee Stock Option [Member] | ||||
| Antidilutive securities excluded from the computation of diluted net loss per share | ||||
| Antidilutive securities excluded from computation of earnings per share (in shares) | 17,336 | 15,473 | 17,336 | 15,473 | 
| Warrant [Member] | ||||
| Antidilutive securities excluded from the computation of diluted net loss per share | ||||
| Antidilutive securities excluded from computation of earnings per share (in shares) | 200 | 200 | 200 | 200 | 
| Employee Stock [Member] | ||||
| Antidilutive securities excluded from the computation of diluted net loss per share | ||||
| Antidilutive securities excluded from computation of earnings per share (in shares) | 103 | 52 | 103 | 52 |