Supplemental balance sheet and cash flow information (trade accounts receivable) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Trade accounts receivable
Trade accounts receivable - gross	$ 128,470	$ 117,991
Allowances for uncollectible accounts	(4,816)	(4,335)	$ (3,861)	$ (3,912)
Trade accounts receivable - net	$ 123,654	$ 113,656