CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock Cumulative Effect, Period of Adoption, as Adjusted	Common Stock	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period of Adoption, as Adjusted	Retained Earnings	Treasury Stock Cumulative Effect, Period of Adoption, as Adjusted	Treasury Stock	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, as Adjusted	Accumulated Other Comprehensive Income (Loss)	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, as Adjusted	Total
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)
Increase (Decrease) in Stockholders' Equity
Net income (loss)					280,372							280,372
Net unrealized appreciation on marketable securities, net of tax									18			18
Exercise of common stock options and vested restricted stock units		$ 2,174										2,174
Exercise of common stock options and vested restricted stock units (in shares)		780
Shares acquired from vested employee stock awards for income tax withholdings							$ (13,556)					(13,556)
Shares acquired from vested employee stock awards for income tax withholdings (in shares)							(239)
Sale of common stock under employee stock purchase plan		$ 3,038										3,038
Sale of common stock under employee stock purchase plan (in shares)		61
Stock based compensation expense		$ 13,105										13,105
Treasury stock purchases							$ (54,448)					(54,448)
Treasury stock purchases (in shares)							(968)
Cash dividends declared					(20,744)							(20,744)
Balance at Dec. 31, 2018	$ 690,910	$ 690,910	$ (13,141)	$ 1,763,444	1,776,585	$ (503,182)	$ (503,182)	$ (32)	(32)	$ (13,141)	$ 1,951,140	1,964,281
Balance (in shares) at Dec. 31, 2018	81,239	81,239				(29,851)	(29,851)
Increase (Decrease) in Stockholders' Equity
Net income (loss)					340,099							340,099
Net unrealized appreciation on marketable securities, net of tax									$ 32			32
Exercise of common stock options and vested restricted stock units		$ 3,106										3,106
Exercise of common stock options and vested restricted stock units (in shares)		707
Shares acquired from vested employee stock awards for income tax withholdings							$ (9,313)					(9,313)
Shares acquired from vested employee stock awards for income tax withholdings (in shares)							(173)
Sale of common stock under employee stock purchase plan		$ 3,165										3,165
Sale of common stock under employee stock purchase plan (in shares)		65
Stock based compensation expense		$ 5,770										5,770
Common stock purchased and cancelled		$ (16,145)										(16,145)
Common stock purchased and cancelled (in shares)		(268)
Treasury stock purchases							$ (78,476)					(78,476)
Treasury stock purchases (in shares)							(1,396)
Cash dividends declared					(24,364)							(24,364)
Balance at Dec. 31, 2019	$ 686,806	$ 686,806	$ (11,639)	$ 2,067,540	2,079,179	$ (590,971)	$ (590,971)			$ (11,639)	$ 2,163,375	2,175,014
Balance (in shares) at Dec. 31, 2019	81,743	81,743				(31,420)	(31,420)
Increase (Decrease) in Stockholders' Equity
Net income (loss)					(8,515)							(8,515)
Exercise of common stock options and vested restricted stock units		$ 38										38
Exercise of common stock options and vested restricted stock units (in shares)		287
Shares acquired from vested employee stock awards for income tax withholdings							$ (6,165)					(6,165)
Shares acquired from vested employee stock awards for income tax withholdings (in shares)							(108)
Sale of common stock under employee stock purchase plan		$ 2,781										2,781
Sale of common stock under employee stock purchase plan (in shares)		65
Stock based compensation expense		$ 6,802										6,802
Treasury stock purchases							$ (20,000)					(20,000)
Treasury stock purchases (in shares)							(386)
Cash dividends declared					(7,019)							(7,019)
Warrants issued under CARES Act agreements		8,248										8,248
Balance at Dec. 31, 2020		$ 704,675			$ 2,052,006		$ (617,136)					$ 2,139,545
Balance (in shares) at Dec. 31, 2020		82,095					(31,914)