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Summary of significant accounting policies - Other Assets (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
Value of common stock | $ $ 14,793 $ 14,788
Visa Class A stock    
Conversion ratio of common stock 1.6483  
Shares to be issued following resolution of Visa litigation | shares 74,142  
Visa Class B stock    
Shares held | shares 44,982  
Value of common stock | $ $ 2,200