CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Capital in Excess of Par Value	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Total
Balance at Dec. 31, 2015	$ 21,699		$ 948,551	$ (17,969)	$ 952,281
Balance (in shares) at Dec. 31, 2015	216,991,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock issued for stock incentive plans, net	$ 80	$ 9,508			9,588
Stock issued for stock incentive plans, net (in shares)	796,000
Stock purchased and retired	$ (30)	(9,935)	6,708		(3,257)
Stock purchased and retired (in shares)	(298,000)
Net income (loss)			(141,246)		(141,246)
Pension adjustment, net of taxes				(788)	(788)
Foreign currency translation				652	652
Unrealized gain (loss) on securities, net of taxes and reclassification adjustment				3	3
Dividends declared			(10,861)		(10,861)
Excess tax benefits for share- based payments		427			427
Balance at Dec. 31, 2016	$ 21,749		803,152	(18,102)	806,799
Balance (in shares) at Dec. 31, 2016	217,489,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock issued for stock incentive plans, net	$ 42	11,048			11,090
Stock issued for stock incentive plans, net (in shares)	420,000
Stock purchased and retired	$ (137)	(11,048)	(15,599)		(26,784)
Stock purchased and retired (in shares)	(1,365,000)
Net income (loss)			162,511		162,511
Pension adjustment, net of taxes				1,033	1,033
Foreign currency translation				391	391
Unrealized gain (loss) on securities, net of taxes and reclassification adjustment				(24)	(24)
Dividends declared			(43,319)		(43,319)
Balance at Dec. 31, 2017	$ 21,654		906,745	(16,702)	$ 911,697
Balance (in shares) at Dec. 31, 2017	216,544,000				216,543,552
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock issued for stock incentive plans, net	$ 37	9,382			$ 9,419
Stock issued for stock incentive plans, net (in shares)	367,000
Stock purchased and retired	$ (237)	$ (9,382)	(33,382)		(43,001)
Stock purchased and retired (in shares)	(2,367,000)
Net income (loss)			175,402		175,402
Pension adjustment, net of taxes				(1,408)	(1,408)
Foreign currency translation				(621)	(621)
Dividends declared			101,069		101,069
Balance at Dec. 31, 2018	$ 21,454		947,711	(18,746)	$ 950,419
Balance (in shares) at Dec. 31, 2018	214,544,000				214,543,511
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of accounting standard (Note 1)			$ 15	$ (15)