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		$ 130,220			130,220
Net unrealized depreciation on marketable securities, net of taxes				33	33
Exercise of common stock options and stock issued under equity award plan	$ 836				836
Exercise of common stock options and stock issued under equity award plan (in shares)	677
Sale of common stock under employee stock purchase plan	$ 1,485				1,485
Sale of common stock under employee stock purchase plan (in shares)	30
Stock-based compensation expense associated with equity awards, net of forfeitures	$ 7,632				7,632
Treasury shares acquired from vested employee stock awards for income tax withholdings			(10,003)		(10,003)
Treasury shares acquired from vested employee stock awards for income tax withholdings (in shares)		(238)
Treasury stock purchases			(13,464)		(13,464)
Treasury stock purchases (in shares)		(177)
Cash dividends declared		$ (10,409)			(10,409)
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)
Balance at Mar. 31, 2018	$ 679,768	$ 1,566,114	(458,645)	(100)	1,787,137
Balance (in shares) at Mar. 31, 2018	81,105	(29,059)
Increase (Decrease) in Stockholders' Equity
Net income		$ 75,859			75,859
Net unrealized depreciation on marketable securities, net of taxes				83	83
Stock-based compensation expense associated with equity awards, net of forfeitures	$ 2,778				2,778
Cash dividends declared		(5,205)			(5,205)
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
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		176,233			176,233
Net unrealized depreciation on marketable securities, net of taxes				$ 32	32
Exercise of common stock options and stock issued under equity award plan	$ 2,920				2,920
Exercise of common stock options and stock issued under equity award plan (in shares)	693
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	$ 1,497				1,497
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			$ (43,476)		(43,476)
Treasury stock purchases (in shares)			(787)
Common stock purchased and cancelled	$ (16,145)				(16,145)
Common stock purchased and cancelled (in shares)	(268)
Cash dividends declared		(12,265)			(12,265)
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 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 shares acquired from vested employee stock awards for income tax withholdings			$ (18,476)		(18,476)
Treasury stock purchases	$ (16,145)				(16,145)
Treasury stock purchases (in shares)			(311)
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)