Commitments and contingencies - Employment agreements and contingent liabilities and acquiring bank sponsorship agreements (Details)		12 Months Ended
	Sep. 01, 2013 USD ($)	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Employment agreements
Commitment to pay severance in event of employment termination without cause		$ 1,200,000	$ 900,000
Commitment to pay severance in event of employment termination upon change of control		$ 1,400,000	$ 1,100,000
Number of acquiring bank sponsoring agreements | item		2
Future minimum payments under acquiring bank sponsorship agreements
2017		$ 416,667
2018		300,000
Total payments		716,667
Fraud related to certain credit card transactions
Employment agreements
Reserves posted by merchants with sponsor bank		900,000
Reserves posted by entity		300,000
Cash reserves required to be posted by merchant, entity		2,200,000
Contingent liability		0
Bank Sponsorship Agreement
Employment agreements
Contingent liability		$ 0
Term of Bank Sponsorship Agreement	5 years
Bank Sponsorship Agreement | Financial Guarantee | Minimum
Employment agreements
Bank sponsorship agreement annual fees in year one	$ 300,000
Bank sponsorship agreement fees final year	500,000
Cost of sales | Bank Sponsorship Agreement | Financial Guarantee | Minimum
Employment agreements
Bank sponsorship agreement fees due	$ 1,800,000