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Subsequent events - Dividend and Stock split (Details) - USD ($)
$ in Thousands
6 Months Ended
Apr. 12, 2018
Feb. 12, 2018
Jun. 30, 2018
Jun. 30, 2017
Subsequent Event        
Dividends paid     $ 1,291,494 $ 0
Stockholders' Equity Note, Stock Split 3022259.23      
Subsequent Event        
Subsequent Event        
Dividends paid   $ 1,112,000    
Stockholders' Equity Note, Stock Split 3,022,259.23