Supplemental balance sheet and cash flow information (inventories and supplies) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Inventories and supplies
Raw materials	$ 5,189		$ 5,412
Semi-finished goods	7,522		7,943
Finished goods	20,481		33,513
Supplies	5,531		5,010
Reserve for excess and obsolete items	(5,456)	$ (7,118)	(11,748)
Inventories and supplies	33,267		$ 40,130
Change in reserve for excess and obsolete items
Balance, beginning of year	11,748	6,600
Amounts charged to expense	2,151	861
Write-offs and sales	(8,443)	(343)
Balance, end of period	$ 5,456	$ 7,118