Nature of operations and summary of significant accounting policies (Inventories) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Energy Related Inventory
Equipment inventory	$ 3,573	$ 3,663
Commodities	521	574
Inventory valuation allowance	(179)	(178)
Inventories, net	$ 3,915	$ 4,059

Amount of valuation reserve for inventory.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant