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, 2022		$ 176	$ (930,643)	$ 1,519,641	$ 3,786,952	$ (456,799)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		2,857
Shares repurchased under share repurchase program (Note 14)		(1)		(20,008)
Repurchases of common stock under incentive plans			(14,389)
Share-based compensation				20,405
Net income	$ 303,844				303,844
Dividends and dividend equivalents declared					(74,314)
Net unrealized gains (losses) on investments, net of tax	32,934					32,934
Other adjustments to other comprehensive income (loss)						179
Balance, end of period at Jun. 30, 2023	4,170,836	177	(945,032)	1,522,895	4,016,482	(423,686)
Balance, beginning of period at Mar. 31, 2023		176	(931,313)	1,515,852	3,908,396	(386,633)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		1		752
Shares repurchased under share repurchase program (Note 14)		0		(4,852)
Repurchases of common stock under incentive plans			(13,719)
Share-based compensation				11,143
Net income	146,087				146,087
Dividends and dividend equivalents declared					(38,001)
Net unrealized gains (losses) on investments, net of tax	(37,053)					(37,053)
Other adjustments to other comprehensive income (loss)						0
Balance, end of period at Jun. 30, 2023	4,170,836	177	(945,032)	1,522,895	4,016,482	(423,686)
Balance, beginning of period at Dec. 31, 2023	4,397,805	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)