CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock Including Additional Paid in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance at Dec. 31, 2012	$ 609,763	$ 1,147,117	$ (371,211)	$ 1,506	$ 1,387,175
Balance (in shares) at Dec. 31, 2012	76,713		(25,280)
Increase (Decrease) in Stockholders' Equity
Net Income (loss)		58,956			58,956
Proportionate share of other companies foreign currency translation adjustment, net of tax of $8 and $678 for 2013, and 2014, respectively				66	66
Net unrealized depreciation on marketable securities net of tax of $43, $437 and $18 for 2013, 2014 and 2015, respectively				(13)	(13)
Exercise of common stock options and issuance of restricted stock	$ 835				835
Exercise of common stock options and issuance of restricted stock (in shares)	313
Sale of common stock under employee stock purchase plan	$ 3,696				3,696
Sale of common stock under employee stock purchase plan (in shares)	300
Stock based compensation expense related to the issuance of stock options and restricted stock	$ 4,363				4,363
Tax deficiency from exercise of common stock options	(146)				(146)
Treasury stock purchases			$ (11,739)		(11,739)
Treasury stock purchases (in shares)			(816)
Cash dividends declared ( $0.16 per share)		(8,254)			(8,254)
Balance at Dec. 31, 2013	$ 618,511	1,197,819	$ (382,950)	1,559	1,434,939
Balance (in shares) at Dec. 31, 2013	77,326		(26,096)
Increase (Decrease) in Stockholders' Equity
Net Income (loss)		(24,154)			(24,154)
Proportionate share of other companies foreign currency translation adjustment, net of tax of $8 and $678 for 2013, and 2014, respectively				(1,129)	(1,129)
Net unrealized depreciation on marketable securities net of tax of $43, $437 and $18 for 2013, 2014 and 2015, respectively				(719)	(719)
Exercise of common stock options and issuance of restricted stock	$ 287				287
Exercise of common stock options and issuance of restricted stock (in shares)	330
Sale of common stock under employee stock purchase plan	$ 3,752				3,752
Sale of common stock under employee stock purchase plan (in shares)	295
Stock based compensation expense related to the issuance of stock options and restricted stock	$ 5,318				5,318
Tax deficiency from exercise of common stock options	(1,347)				(1,347)
Treasury stock purchases			$ (8,414)		(8,414)
Treasury stock purchases (in shares)			(669)
Cash dividends declared ( $0.16 per share)		(8,187)			(8,187)
Balance at Dec. 31, 2014	$ 626,521	1,165,478	$ (391,364)	(289)	1,400,346
Balance (in shares) at Dec. 31, 2014	77,951		(26,765)
Increase (Decrease) in Stockholders' Equity
Net Income (loss)		117,817			117,817
Net unrealized depreciation on marketable securities net of tax of $43, $437 and $18 for 2013, 2014 and 2015, respectively				29	29
Exercise of common stock options and issuance of restricted stock	$ 8,490				8,490
Exercise of common stock options and issuance of restricted stock (in shares)	815
Sale of common stock under employee stock purchase plan	$ 3,430				3,430
Sale of common stock under employee stock purchase plan (in shares)	254
Stock based compensation expense related to the issuance of stock options and restricted stock	$ 5,368				5,368
Tax deficiency from exercise of common stock options	(2,166)				(2,166)
Treasury stock purchases			$ (18,726)		(18,726)
Treasury stock purchases (in shares)			(1,250)
Cash dividends declared ( $0.16 per share)		(8,153)			(8,153)
Balance at Dec. 31, 2015	$ 641,643	$ 1,275,142	$ (410,090)	$ (260)	$ 1,506,435
Balance (in shares) at Dec. 31, 2015	79,020		(28,015)