Summary of significant accounting policies (Details 6) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Segment and geographical information
Number of reportable segments									1
Revenue	$ 28,000	$ 26,017	$ 24,302	$ 24,187	$ 25,899	$ 24,688	$ 22,943	$ 21,028	$ 102,506	$ 94,558	$ 80,420
Retail subscription
Segment and geographical information
Revenue									32,716	29,033	23,561
Retail single-use
Segment and geographical information
Revenue									13,429	16,054	17,460
Wholesale
Segment and geographical information
Revenue									49,495	43,707	35,134
Advertising and other
Segment and geographical information
Revenue									$ 6,866	$ 5,764	$ 4,265