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Nature of operations and summary of significant accounting policies (Inventories) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Mar. 21, 2017
Dec. 31, 2017
Dec. 31, 2019
Dec. 31, 2018
Accounting Policies [Abstract]        
Equipment inventory     $ 3,435 $ 3,663
Commodities     474 574
Inventory valuation allowance     (179) (178)
Inventories, net $ 6,885   3,730 4,059
Inventory - valuation adjustment $ 0 $ 179 $ 7,188 $ 0