Stockholders' equity (Details) - shares shares in Thousands	Dec. 31, 2015	Dec. 31, 2014
Stockholders' equity (deficit)
Common stock authorized (in shares)	100,000	100,000
Shares of Common Stock reserved
Outstanding stock options (in shares)	3,748	4,341
Total (in shares)	11,545	10,218
2001 Plan
Shares of Common Stock reserved
Outstanding stock options (in shares)	1,220	1,525
2011 Plan
Shares of Common Stock reserved
Outstanding stock options (in shares)	2,528	2,816
Outstanding RSUs (in shares)	1,819	1,385
Number of shares available for grant (in shares)	5,978	4,492