Condensed Consolidated Statements of Changes in Common Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance, beginning of period at Dec. 31, 2023		$ 173	$ (945,870)	$ 1,430,594	$ 4,243,759	$ (330,851)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		2,387
Shares repurchased under share repurchase program (Note 14)		(3)		(100,680)
Repurchases of common stock under incentive plans			(21,348)
Share-based compensation				24,040
Net income	$ 304,257				304,257
Dividends and dividend equivalents declared					(77,681)
Net unrealized gains (losses) on investments, net of tax	(46,392)					(46,392)
Other adjustments to other comprehensive income (loss)						(68)
Balance, end of period at Jun. 30, 2024	4,482,319	172	(967,218)	1,356,341	4,470,335	(377,311)
Balance, beginning of period at Mar. 31, 2024		171	(946,202)	1,390,436	4,357,823	(362,496)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		856
Shares repurchased under share repurchase program (Note 14)		(1)		(50,179)
Repurchases of common stock under incentive plans			(21,016)
Share-based compensation				15,228
Net income	151,903				151,903
Dividends and dividend equivalents declared					(39,391)
Net unrealized gains (losses) on investments, net of tax	(14,815)					(14,815)
Other adjustments to other comprehensive income (loss)						0
Balance, end of period at Jun. 30, 2024	4,482,319	172	(967,218)	1,356,341	4,470,335	(377,311)
Balance, beginning of period at Dec. 31, 2024	4,623,858	168	(968,246)	1,246,826	4,695,348	(350,238)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		1,838
Shares repurchased under share repurchase program (Note 14)		(13)		(433,967)
Repurchases of common stock under incentive plans			(20,518)
Share-based compensation				32,702
Net income	286,354				286,354
Dividends and dividend equivalents declared					(74,872)
Net unrealized gains (losses) on investments, net of tax	77,252					77,252
Other adjustments to other comprehensive income (loss)						45
Balance, end of period at Jun. 30, 2025	4,492,681	157	(988,764)	847,399	4,906,830	(272,941)
Balance, beginning of period at Mar. 31, 2025		162	(969,396)	1,048,738	4,802,038	(294,716)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		668
Shares repurchased under share repurchase program (Note 14)		(7)		(224,823)
Repurchases of common stock under incentive plans			(19,368)
Share-based compensation				22,816
Net income	141,796				141,796
Dividends and dividend equivalents declared					(37,004)
Net unrealized gains (losses) on investments, net of tax	21,775					21,775
Other adjustments to other comprehensive income (loss)						0
Balance, end of period at Jun. 30, 2025	$ 4,492,681	$ 157	$ (988,764)	$ 847,399	$ 4,906,830	$ (272,941)