Supplemental balance sheet information (allowances for uncollectible accounts, inventories and supplies) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Allowances for uncollectible accounts
Balance, beginning of year	$ 2,884	$ 2,828
Bad debt expense	875	692
Write-offs, net of recoveries	(905)	(634)
Balance, end of period	2,854	$ 2,886
Inventories and supplies
Raw materials	6,949		$ 7,357
Semi-finished goods	7,633		7,635
Finished goods	24,724		24,146
Supplies	3,078		3,111
Inventories and supplies	$ 42,384		$ 42,249