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, 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		428,907			428,907
Net unrealized appreciation on marketable securities, net of taxes				21	21
Exercise of common stock options and stock issued under equity award plan	$ 1,658				1,658
Exercise of common stock options and stock issued under equity award plan (in shares)	529
Treasury shares acquired from vested employee stock awards for income tax withholdings			$ (5,080)		(5,080)
Treasury shares acquired from vested employee stock awards for income tax withholdings (in shares)			(145)
Sale of common stock under employee stock purchase plan	$ 3,002				3,002
Sale of common stock under employee stock purchase plan (in shares)	88
Stock based compensation expense related to the issuance of equity awards	$ 10,580				10,580
Impact of adoption of ASU 2016-09 (See Note 1) | Accounting Standards Update 2016-09		867			867
Treasury stock purchases			$ (20,008)		(20,008)
Treasury stock purchases (in shares)			(484)
Cash dividends declared		(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)
Increase (Decrease) in Stockholders' Equity
Net income		280,372			280,372
Net unrealized appreciation on marketable securities, net of taxes				18	18
Exercise of common stock options and stock issued under equity award plan	$ 2,174				2,174
Exercise of common stock options and stock issued under equity award plan (in shares)	780
Treasury shares acquired from vested employee stock awards for income tax withholdings			$ (13,556)		(13,556)
Treasury 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 related to the issuance of equity awards	$ 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	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
Change in accounting principle and other (see Note 6) | Accounting Standards Update 2016-02		(13,141)			(13,141)
Balance, as adjusted		1,763,444			1,951,140
Net income		340,099			340,099
Net unrealized appreciation on marketable securities, net of taxes				$ 32	32
Exercise of common stock options and stock issued under equity award plan	$ 3,106				3,106
Exercise of common stock options and stock issued under equity award plan (in shares)	707
Treasury shares acquired from vested employee stock awards for income tax withholdings			$ (9,313)		(9,313)
Treasury 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 related to the issuance of equity awards	$ 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	$ 2,079,179	$ (590,971)		$ 2,175,014
Balance (in shares) at Dec. 31, 2019	81,743		(31,420)