CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Total
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 $XXX and $678 for 2015 and 2014, respectively				(1,129)	(1,129)
Net unrealized depreciation on marketable securities net of tax of $98, $18 and $437 for 2016, 2015 and 2014, respectively				(719)	(719)
Exercise of common stock options and vested restricted stock units	$ 287				287
Exercise of common stock options and vested restricted stock units (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 units	$ 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 $98, $18 and $437 for 2016, 2015 and 2014, respectively				29	29
Exercise of common stock options and vested restricted stock units	$ 8,490				8,490
Exercise of common stock options and vested restricted stock units (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 units	$ 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)
Increase (Decrease) in Stockholders' Equity
Net income (loss)		(161,586)			(161,586)
Net unrealized depreciation on marketable securities net of tax of $98, $18 and $437 for 2016, 2015 and 2014, respectively				189	189
Exercise of common stock options and vested restricted stock units	$ 4,979				4,979
Exercise of common stock options and vested restricted stock units (in shares)	609
Sale of common stock under employee stock purchase plan	$ 3,163				3,163
Sale of common stock under employee stock purchase plan (in shares)	152
Stock based compensation expense related to the issuance of stock options and restricted stock units	$ 7,568				7,568
Cash dividends declared ( $0.16 per share)		(9,805)			(9,805)
Balance at Dec. 31, 2016	$ 657,353	$ 1,103,751	$ (410,090)	$ (71)	$ 1,350,943
Balance (in shares) at Dec. 31, 2016	79,781		(28,015)