Summary of significant accounting policies (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 item	Jun. 30, 2013	Dec. 31, 2013
Primary revenue source
Number of reportable segments			1
Revenue	$ 28,396	$ 26,239	$ 54,848	$ 49,373
Marketable securities
Short-term marketable securities	29,909		29,909		32,962
Wholesale
Primary revenue source
Revenue	12,872	12,339	23,995	23,894
Retail subscription
Primary revenue source
Revenue	8,161	8,731	16,455	16,798
Retail single-use
Primary revenue source
Revenue	2,830	2,830	5,366	5,416
Advertising and other
Primary revenue source
Revenue	$ 4,533	$ 2,339	$ 9,032	$ 3,265