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		54,362			54,362
Net unrealized depreciation on marketable securities, net of taxes				(50)	(50)
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
Stock-based compensation expense (forfeiture credit) associated with equity awards, net	$ 4,854				4,854
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
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 (per share)		(5,205)			(5,205)
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
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		88,181			88,181
Net unrealized depreciation on marketable securities, net of taxes				$ 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
Stock-based compensation expense (forfeiture credit) associated with equity awards, net	$ (578)				(578)
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
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 (per share)		(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)