CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
CURRENT ASSETS:
Accounts receivable, allowances for discounts and doubtful accounts	$ 6,090	$ 5,687
Property, plant and equipment, accumulated depreciation	$ 192,515	$ 186,135
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)	1,629,398	1,503,284