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Investment in equity affiliate (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 12, 2017
Schedule of Equity Method Investments [Line Items]          
Loss from equity-method investment $ (58) $ 0 $ (114) $ 0  
Corridor          
Schedule of Equity Method Investments [Line Items]          
Aggregate cost         $ 3,000
Loss from equity-method investment $ 58   $ 114