Summary of significant accounting policies (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 item	Mar. 31, 2013	Dec. 31, 2013
Primary revenue source
Number of reportable segments	1
Revenue	$ 26,452	$ 23,134
Marketable securities
Short-term marketable securities	37,286		32,962
Unrealized gains or losses from investments in marketable securities	0
Realized gains or losses from investments in marketable securities	0
Wholesale
Primary revenue source
Revenue	11,123	11,555
Retail subscription
Primary revenue source
Revenue	8,294	8,067
Retail single-use
Primary revenue source
Revenue	2,536	2,586
Advertising and other
Primary revenue source
Revenue	$ 4,499	$ 926