CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 1,184	$ 1,354
Property, plant and equipment, accumulated depreciation	$ 323,623	$ 317,414
Capital Stock, par value (in dollars per share)	$ 1	$ 1
Capital Stock, authorized shares (shares)	50,000,000	50,000,000
Capital Stock, shares issued (shares)	18,546,000	18,395,000
Capital Stock, shares outstanding (shares)	18,546,000	18,395,000