CONSOLIDATED STATEMENTS OF Financial Position (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Trade accounts receivable, allowances	$ 1,914	$ 1,921	$ 1,937
Treasury stock, shares	88,000	85,000	85,000
Common Stock
Common stock, par value (in dollars per share)	$ 0.6944	$ 0.6944	$ 0.6944
Common stock, shares authorized	120,000,000	120,000,000	120,000,000
Common stock, shares issued	38,724,000	37,960,000	38,580,000
Class B Common Stock
Common stock, par value (in dollars per share)	$ 0.6944	$ 0.6944	$ 0.6944
Common stock, shares authorized	40,000,000	40,000,000	40,000,000
Common stock, shares issued	25,637,000	24,891,000	24,932,000