CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Total
Increase (Decrease) in Stockholders' Equity
Change in accounting principle and other		$ (13,141)			$ (13,141)
Balance, as adjusted	$ 690,910	1,763,444	$ (503,182)	$ (32)	1,951,140
Balance at Dec. 31, 2018	$ 690,910	1,776,585	$ (503,182)	(32)	1,964,281
Balance (in shares) at Dec. 31, 2018	81,239		(29,851)
Increase (Decrease) in Stockholders' Equity
Net income (loss)		88,181			88,181
Net unrealized appreciation on marketable securities, net of tax				32	32
Exercise of common stock options and stock issued under equity award plan	$ 641				641
Exercise of common stock options and stock issued under equity award plan (in shares)	521
Sale of common stock under employee stock purchase plan	$ 1,620				1,620
Sale of common stock under employee stock purchase plan (in shares)	39
Stock-based compensation expense (forfeiture credit) associated with equity awards, net	$ (578)				(578)
Treasury shares acquired from vested employee stock awards for income tax withholdings			$ (9,311)		(9,311)
Treasury shares acquired from vested employee stock awards for income tax withholdings (in shares)			(173)
Treasury stock purchases			$ (25,000)		(25,000)
Treasury stock purchases (in shares)			(476)
Cash dividends declared		(6,158)			(6,158)
Balance at Mar. 31, 2019	$ 692,593	1,845,467	$ (537,493)		2,000,567
Balance (in shares) at Mar. 31, 2019	81,799		(30,500)
Balance at Dec. 31, 2018	$ 690,910	1,776,585	$ (503,182)	(32)	1,964,281
Balance (in shares) at Dec. 31, 2018	81,239		(29,851)
Increase (Decrease) in Stockholders' Equity
Net income (loss)					176,233
Balance at Jun. 30, 2019	$ 680,802	1,927,412	$ (555,969)		2,052,245
Balance (in shares) at Jun. 30, 2019	81,703		(30,811)
Balance at Dec. 31, 2018	$ 690,910	1,776,585	$ (503,182)	(32)	1,964,281
Balance (in shares) at Dec. 31, 2018	81,239		(29,851)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	$ 81,743
Balance at Dec. 31, 2019	$ 686,806	2,079,179	$ (590,971)		2,175,014
Balance (in shares) at Dec. 31, 2019	81,743		(31,420)
Balance at Mar. 31, 2019	$ 692,593	1,845,467	$ (537,493)		2,000,567
Balance (in shares) at Mar. 31, 2019	81,799		(30,500)
Increase (Decrease) in Stockholders' Equity
Net income (loss)		88,052			88,052
Exercise of common stock options and stock issued under equity award plan	$ 2,279				2,279
Exercise of common stock options and stock issued under equity award plan (in shares)	172
Stock-based compensation expense (forfeiture credit) associated with equity awards, net	$ 2,075				2,075
Treasury stock purchases			$ (18,476)		(18,476)
Treasury stock purchases (in shares)			(311)
Common stock purchased and cancelled	$ (16,145)				(16,145)
Common stock purchased and cancelled (in shares)	(268)
Cash dividends declared		(6,107)			(6,107)
Balance at Jun. 30, 2019	$ 680,802	1,927,412	$ (555,969)		2,052,245
Balance (in shares) at Jun. 30, 2019	81,703		(30,811)
Increase (Decrease) in Stockholders' Equity
Change in accounting principle and other		(11,639)			(11,639)
Balance, as adjusted	$ 686,806	2,067,540	$ (590,971)		2,163,375
Balance at Dec. 31, 2019	$ 686,806	2,079,179	$ (590,971)		2,175,014
Balance (in shares) at Dec. 31, 2019	81,743		(31,420)
Increase (Decrease) in Stockholders' Equity
Net income (loss)		29,988			29,988
Exercise of common stock options and stock issued under equity award plan	$ 38				38
Exercise of common stock options and stock issued under equity award plan (in shares)	287
Sale of common stock under employee stock purchase plan	$ 1,494				1,494
Sale of common stock under employee stock purchase plan (in shares)	24
Stock-based compensation expense (forfeiture credit) associated with equity awards, net	$ 1,727				1,727
Treasury shares acquired from vested employee stock awards for income tax withholdings			$ (6,165)		(6,165)
Treasury shares acquired from vested employee stock awards for income tax withholdings (in shares)			(108)
Treasury stock purchases			$ (20,000)		(20,000)
Treasury stock purchases (in shares)			(386)
Cash dividends declared		(7,019)			(7,019)
Balance at Mar. 31, 2020	$ 690,065	2,090,509	$ (617,136)		2,163,438
Balance (in shares) at Mar. 31, 2020	82,054		(31,914)
Balance at Dec. 31, 2019	$ 686,806	2,079,179	$ (590,971)		2,175,014
Balance (in shares) at Dec. 31, 2019	81,743		(31,420)
Increase (Decrease) in Stockholders' Equity
Net income (loss)					4,273
CARES Act warrant issuance					2,845
Balance at Jun. 30, 2020	$ 694,997	2,064,794	$ (617,136)	307	2,142,962
Balance (in shares) at Jun. 30, 2020	82,054		(31,914)
Balance at Mar. 31, 2020	$ 690,065	2,090,509	$ (617,136)		2,163,438
Balance (in shares) at Mar. 31, 2020	82,054		(31,914)
Increase (Decrease) in Stockholders' Equity
Net income (loss)		(25,715)			(25,715)
Net unrealized appreciation on marketable securities, net of tax				307	307
Stock-based compensation expense (forfeiture credit) associated with equity awards, net	$ 2,087				2,087
CARES Act warrant issuance	2,845				2,845
Balance at Jun. 30, 2020	$ 694,997	$ 2,064,794	$ (617,136)	$ 307	$ 2,142,962
Balance (in shares) at Jun. 30, 2020	82,054		(31,914)