Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accounts receivable, allowance for doubtful accounts	$ 813	$ 430
Property, plant and equipment, accumulated depreciation	$ 131,251	$ 133,559
Preferred stock, preferred stock authorized (in shares)	1,000,000	1,000,000
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	25,000,000	25,000,000
Common stock, shares issued (in shares)	12,121,484	12,115,055
Common stock, shares outstanding (in shares)	6,276,963	6,260,075
Treasury stock, shares (in shares)	5,844,521	5,854,980
Series A Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, Series A convertible shares authorized (in shares)	9,926	9,926
Preferred stock, Series A convertible shares issued (in shares)	9,926	9,926
Preferred stock, Series A convertible shares outstanding (in shares)	9,926	9,926