Consolidated Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Accounts receivable, allowances for doubtful accounts	$ 2,008	$ 1,772
Stockholders' equity:
Preferred stock, par value (in dollars per share)	$ 1.00	$ 1.00
Preferred stock, shares authorized (in shares)	1,000,000	1,000,000
Preferred stock, shares issued	0	0
Common Stock [Member]
Stockholders' equity:
Common stock, par value (in dollars per share)	$ 0.10	$ 0.10
Common stock, shares authorized (in shares)	300,000,000	300,000,000
Common stock, shares outstanding (in shares)	131,874,587	133,846,801
Common Class B [Member]
Stockholders' equity:
Common stock, par value (in dollars per share)	$ 0.10	$ 0.10
Common stock, shares authorized (in shares)	40,000,000	40,000,000
Common stock, shares outstanding (in shares)	12,129,227	12,129,227