Supplemental balance sheet and cash flow information (trade accounts receivable, inventories and supplies) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Trade accounts receivable
Trade accounts receivable - gross	$ 91,910	$ 74,299
Allowances for uncollectible accounts	(3,861)	(3,912)	(4,007)
Trade accounts receivable - net	88,049	70,387
Allowances for uncollectible accounts
Balance, beginning of year	3,912	4,007	4,130
Bad debt expense	3,722	3,749	4,033
Write-offs, net of recoveries	(3,773)	(3,844)	(4,156)
Balance, end of year	3,861	3,912	4,007
Inventories and supplies
Raw materials	5,426	4,818
Semi-finished goods	8,361	8,390
Finished goods	11,948	7,005
Supplies	3,231	3,078
Inventories and supplies	$ 28,966	$ 23,291