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, 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					88,181							88,181
Net unrealized depreciation 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 associated with equity awards, net of forfeitures		$ (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	$ 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												267,572
Balance at Sep. 30, 2019		$ 684,541			2,012,691		$ (580,972)					2,116,260
Balance (in shares) at Sep. 30, 2019		81,743					(31,253)
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					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 associated with equity awards, net of forfeitures		$ 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
Net income					91,339							91,339
Exercise of common stock options and stock issued under equity award plan		$ 185										185
Exercise of common stock options and stock issued under equity award plan (in shares)		13
Sale of common stock under employee stock purchase plan		$ 1,545										1,545
Sale of common stock under employee stock purchase plan (in shares)		27
Stock-based compensation expense associated with equity awards, net of forfeitures		$ 2,009										2,009
Treasury stock purchases							$ (25,003)					(25,003)
Treasury stock purchases (in shares)							(442)
Cash dividends declared					(6,060)							(6,060)
Balance at Sep. 30, 2019		$ 684,541			2,012,691		$ (580,972)					2,116,260
Balance (in shares) at Sep. 30, 2019		81,743					(31,253)
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					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 associated with equity awards, net of forfeitures		$ 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	$ 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												37,935
CARES Act warrant issuance												8,248
Balance at Sep. 30, 2020		$ 703,780			2,098,456		$ (617,136)					2,185,100
Balance (in shares) at Sep. 30, 2020		82,095					(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					(25,715)							(25,715)
Net unrealized appreciation on marketable securities, net of tax									307			307
Stock-based compensation expense associated with equity awards, net of forfeitures		$ 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)
Increase (Decrease) in Stockholders' Equity
Net income					33,662							33,662
Net unrealized depreciation on marketable securities, net of tax									$ (307)			(307)
Sale of common stock under employee stock purchase plan		$ 1,287										1,287
Sale of common stock under employee stock purchase plan (in shares)		41
Stock-based compensation expense associated with equity awards, net of forfeitures		$ 2,093										2,093
CARES Act warrant issuance		5,403										5,403
Balance at Sep. 30, 2020		$ 703,780			$ 2,098,456		$ (617,136)					$ 2,185,100
Balance (in shares) at Sep. 30, 2020		82,095					(31,914)