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, 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		$ 213,266			213,266
Net unrealized depreciation on marketable securities, net of taxes				32	32
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
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 associated with equity awards, net of forfeitures	$ 10,363				10,363
Treasury shares acquired from vested employee stock awards for income tax withholdings			(25,164)		(25,164)
Treasury shares acquired from vested employee stock awards for income tax withholdings (in shares)		(428)
Treasury stock purchases			(13,556)		(13,556)
Treasury stock purchases (in shares)		(239)
Cash dividends declared		$ (15,602)			(15,602)
Balance at Sep. 30, 2018	$ 688,168	$ 1,714,621	(473,898)	(18)	1,928,873
Balance (in shares) at Sep. 30, 2018	81,239	(29,311)
Balance at Jun. 30, 2018	$ 682,546	$ 1,636,768	(458,645)	(17)	1,860,652
Balance (in shares) at Jun. 30, 2018	81,105	(29,059)
Increase (Decrease) in Stockholders' Equity
Net income		$ 83,046			83,046
Net unrealized depreciation on marketable securities, net of taxes				(1)	(1)
Exercise of common stock options and stock issued under equity award plan	$ 1,338				1,338
Exercise of common stock options and stock issued under equity award plan (in shares)	103
Sale of common stock under employee stock purchase plan	$ 1,553				1,553
Sale of common stock under employee stock purchase plan (in shares)	31
Stock-based compensation expense associated with equity awards, net of forfeitures	$ 2,731				2,731
Treasury shares acquired from vested employee stock awards for income tax withholdings			(15,161)		(15,161)
Treasury shares acquired from vested employee stock awards for income tax withholdings (in shares)		(250)
Treasury stock purchases			(92)		(92)
Treasury stock purchases (in shares)		(2)
Cash dividends declared		$ (5,193)			(5,193)
Balance at Sep. 30, 2018	$ 688,168	$ 1,714,621	(473,898)	(18)	1,928,873
Balance (in shares) at Sep. 30, 2018	81,239	(29,311)
Increase (Decrease) in Stockholders' Equity
Change in accounting principle and other (see Note 6)		$ (13,141)			(13,141)
Balance at December 31, 2018, 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		267,572			267,572
Net unrealized depreciation on marketable securities, net of taxes				$ 32	32
Exercise of common stock options and stock issued under equity award plan	$ 3,105				3,105
Exercise of common stock options and stock issued under equity award plan (in shares)	707
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 associated with equity awards, net of forfeitures	$ 3,506				3,506
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)
Treasury stock purchases			$ (68,477)		(68,477)
Treasury stock purchases (in shares)			(1,229)
Common stock purchased and cancelled	$ (16,145)				(16,145)
Common stock purchased and cancelled (in shares)	(268)
Cash dividends declared		(18,325)			(18,325)
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 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 shares acquired from vested employee stock awards for income tax withholdings			$ (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)