Condensed Consolidated Statements Of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid-In Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	Available-for-sale-securities [Member]	Foreign Currency Translation [Member]	Total
Balance at Dec. 31, 2018	$ 360	$ 1,374,154	$ (2,500,556)	$ 2,573,617	$ (147)	$ (6,089)	$ 1,441,339
Balance, Shares at Dec. 31, 2018	35,973		8,276
Stock-based compensation	$ 0	19,342	$ 0	0	0	0	19,342
Stock-based compensation, Shares	0		0
Stock plan transactions and other	$ 1	(212)	$ 0	0	0	0	(211)
Stock plan transactions and other, Shares	135		0
Acquisition of treasury stock (value)	$ 0	0	$ (62,854)	0	0	0	(62,854)
Acquisition of treasury stock (shares)	0		110
Net income	$ 0	0	$ 0	88,132	0	0	88,132
Other comprehensive income (loss), net of income tax	0	0	0	0	100	278	378
Balance at Mar. 31, 2019	$ 361	1,393,284	$ (2,563,410)	2,659,422	(47)	(5,811)	1,483,799
Balance, Shares at Mar. 31, 2019	36,108		8,386
Adoption of accounting standard | Accounting Standards Update 2016-02 [Member]	$ 0	0	$ 0	(2,327)	0	0	(2,327)
Balance at Dec. 31, 2019	$ 363	1,465,697	$ (2,699,119)	2,921,448	0	(5,363)	1,683,026
Balance, Shares at Dec. 31, 2019	36,323		8,568
Stock-based compensation	$ 0	17,708	$ 0	0	0	0	17,708
Stock-based compensation, Shares	0		0
Stock plan transactions and other	$ 2	(181)	$ 0	0	0	0	(179)
Stock plan transactions and other, Shares	194		0
Acquisition of treasury stock (value)	$ 0	0	$ (102,031)	0	0	0	(102,031)
Acquisition of treasury stock (shares)	0		134
Net income	$ 0	0	$ 0	76,388	0	0	76,388
Other comprehensive income (loss), net of income tax	0	0	0	0	0	(1,841)	(1,841)
Balance at Mar. 31, 2020	$ 365	1,483,224	$ (2,801,150)	2,996,785	0	(7,204)	1,672,020
Balance, Shares at Mar. 31, 2020	36,517		8,702
Adoption of accounting standard | Accounting Standards Update 2016-13 [Member]	$ 0	$ 0	$ 0	$ (1,051)	$ 0	$ 0	$ (1,051)