Supplemental balance sheet and cash flow information (allowances for uncollectible accounts, inventories and supplies) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Accounts receivable, allowance for uncollectible accounts [Roll Forward]
Balance, beginning of year	$ 4,985	$ 3,639
Bad debt expense	1,059	1,248
Write-offs and other	(2,098)	(423)
Balance, end of period	3,946	$ 4,464
Inventories and supplies
Raw materials	7,355		$ 6,977
Semi-finished goods	7,532		7,368
Finished goods	24,091		21,982
Supplies	3,703		3,594
Inventories and supplies	$ 42,681		$ 39,921

Amount of allowance for credit loss on accounts receivable, classified as current.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of direct write-downs of accounts receivable charged against the allowance.

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Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of merchandise or goods held by the company that are readily available for sale.

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Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

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Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of unprocessed items to be consumed in the manufacturing or production process.

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Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of products used directly or indirectly in the manufacturing or production process, which may or may not become part of the final product. May also include items used in the storage, presentation or transportation of physical goods.

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Carrying amount, net of reserves and adjustments, as of the balance sheet date of merchandise or goods which are partially completed. This inventory is generally comprised of raw materials, labor and factory overhead costs, which require further materials, labor and overhead to be converted into finished goods, and which generally require the use of estimates to determine percentage complete and pricing.

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Amount of expense (reversal of expense) for expected credit loss on accounts receivable.

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