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, 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 appreciation on marketable securities net of tax				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 (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 appreciation on marketable securities net of tax				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 (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)
Increase (Decrease) in Stockholders' Equity
Net income (loss)		428,907			428,907
Net unrealized appreciation on marketable securities net of tax				21	21
Exercise of common stock options and vested restricted stock units	$ 1,658				1,658
Exercise of common stock options and vested restricted stock units (in shares)	529
Treasury shares withheld for the payment of taxes			$ (5,080)		(5,080)
Treasury shares withheld for the payment of taxes (in shares)			(145)
Sale of common stock under employee stock purchase plan (in shares)	88
Stock based compensation expense related to the issuance of stock options and restricted stock units	$ 3,002				3,002
Impact of adoption of Accounting Standards Update (ASU) 2016-09 (See Note 1)		867			867
Tax deficiency from exercise of common stock options	10,580				10,580
Treasury stock purchases			$ (20,008)		(20,008)
Treasury stock purchases (in shares)			(484)
Cash dividends declared (per share)		(16,568)			(16,568)
Balance at Dec. 31, 2017	$ 672,593	$ 1,516,957	$ (435,178)	$ (50)	$ 1,754,322
Balance (in shares) at Dec. 31, 2017	80,398		(28,644)