Supplemental balance sheet and cash flow information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Trade accounts receivable [Abstract]
Trade accounts receivable - gross	$ 73,030	$ 70,601
Allowances for uncollectible accounts	(4,007)	(4,130)	(4,991)
Trade accounts receivable - net	69,023	66,471
Allowances for uncollectible accounts [Abstract]
Balance, beginning of year	4,130	4,991	5,930
Bad debt expense	4,033	4,686	5,842
Write-offs, net of recoveries	(4,156)	(5,547)	(6,781)
Balance, end of year	4,007	4,130	4,991
Inventories and supplies [Abstract]
Raw materials	5,566	4,879
Semi-finished goods	8,273	8,393
Finished goods	5,301	5,083
Supplies, primarily production	2,903	3,305
Inventories and supplies	$ 22,043	$ 21,660