Summary of significant accounting policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Revenue recognition
Number of revenue streams | item	2
Payment processing services revenue
Provision for sales credits	$ 0.0	$ 0.0	$ 0.0
Translation of non-U.S. currencies
Amounts resulting from foreign currency transactions included in statement of operations	0.4	0.2	$ 0.1
Allowance for doubtful accounts
Allowance for doubtful accounts	$ 0.1	$ 0.1