CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
CURRENT ASSETS:
Accounts receivable, allowances for discounts and doubtful accounts	$ 5,533	$ 4,967
Property, plant and equipment, accumulated depreciation	$ 186,603	$ 191,081
Stockholders' equity:
Common stock, par value (in dollars per share)	$ 2.00	$ 2.00
Common stock, shares authorized (in shares)	30,000,000	30,000,000
Common stock, shares issued (in shares)	23,936,036	23,936,036
Treasury stock - at cost (in shares)	1,490,526	1,424,025