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Other Current Assets (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]    
Prepayments to suppliers $ 5,869 $ 4,056
Other debtors 3,477 4,762
Prepaid expenses 9,405 12,295
Derivative instruments 0 1,703
Gross book value at end of period 18,751 22,816
Net book value at end of period $ 18,751 $ 22,816