CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Trade accounts receivable, allowances	$ 1,820	$ 1,921
Treasury stock, shares	88	85
Common Stock
Common stock, par value (in dollars per share)	$ 0.6944	$ 0.6944
Common stock, shares authorized	120,000	120,000
Common stock, shares issued	38,544	37,960
Class B Common Stock
Common stock, par value (in dollars per share)	$ 0.6944	$ 0.6944
Common stock, shares authorized	40,000	40,000
Common stock, shares issued	25,584	24,891