Summary of significant accounting policies (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Revenue recognition
Obligations related to the earned fees and collection of the related receivable	0
Platform service arrangements | Minimum
Revenue recognition
Term of the arrangement used to determine revenue recognition	2 years
Platform service arrangements | Maximum
Revenue recognition
Term of the arrangement used to determine revenue recognition	5 years
Wholesale partner arrangement | Minimum
Revenue recognition
Term of the arrangement used to determine revenue recognition	3 years
Wholesale partner arrangement | Maximum
Revenue recognition
Term of the arrangement used to determine revenue recognition	15 years