CONSOLIDATED STATEMENTS OF Financial Position (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Trade accounts receivable, allowances	$ 1,921	$ 1,884
Treasury stock, shares	85	83
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	37,960	37,701
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	24,891	24,221