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		88,181			88,181
Net unrealized depreciation on marketable securities, net of tax of $10				$ 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)
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		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)