Segment information - Concentration of revenue by customer by geographical region (Details) - Revenue	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Geographical concentration risk | Multi-currency processing services | APAC | Customer A
Revenue concentration
Concentration risk (as a percent)	59.00%	56.00%	58.00%	56.00%
Geographical concentration risk | Multi-currency processing services | The Americas | Customer D
Revenue concentration
Concentration risk (as a percent)	12.00%	22.00%	16.00%	22.00%
Geographical concentration risk | Multi-currency processing services | The Americas | Customer E
Revenue concentration
Concentration risk (as a percent)	10.00%
Geographical concentration risk | Multi-currency processing services | The Americas | Customer F
Revenue concentration
Concentration risk (as a percent)	10.00%
Geographical concentration risk | Multi-currency processing services | The Americas | Customer G
Revenue concentration
Concentration risk (as a percent)			11.00%
Geographical concentration risk | Multi-currency processing services | The Americas | Customer I
Revenue concentration
Concentration risk (as a percent)	11.00%	15.00%	12.00%	12.00%
Geographical concentration risk | Multi-currency processing services | The Americas | Customer J
Revenue concentration
Concentration risk (as a percent)	11.00%	15.00%	10.00%	11.00%
Geographical concentration risk | Multi-currency processing services | EMEA | Customer K
Revenue concentration
Concentration risk (as a percent)	28.00%		15.00%
Geographical concentration risk | Multi-currency processing services | EMEA | Customer C
Revenue concentration
Concentration risk (as a percent)	36.00%	58.00%	44.00%	61.00%
Geographical concentration risk | Multi-currency processing services | EMEA | Customer H
Revenue concentration
Concentration risk (as a percent)	34.00%	37.00%	40.00%	37.00%
Customer concentration risk | Customer A
Revenue concentration
Concentration risk (as a percent)	18.00%	16.00%	19.00%	16.00%
Customer concentration risk | Customer G
Revenue concentration
Concentration risk (as a percent)		14.00%		12.00%
Customer concentration risk | Multi-currency processing services | Customer G
Revenue concentration
Concentration risk (as a percent)	14.00%	35.00%	18.00%	30.00%
Customer concentration risk | Multi-currency processing services | Customer B
Revenue concentration
Concentration risk (as a percent)	22.00%	14.00%	22.00%	15.00%