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, 2021		$ 194	$ (920,798)	$ 1,878,372	$ 3,180,935	$ 120,093
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		1,638
Shares repurchased under share repurchase program (Note 14)		(10)		(205,086)
Repurchases of common stock under incentive plans			(9,486)
Share-based compensation				23,566
Net income	$ 382,324				382,324
Dividends and dividend equivalents declared					(71,584)
Net unrealized gains (losses) on investments, net of tax	(449,282)					(449,282)
Other adjustments to other comprehensive income (loss)						84
Balance, end of period at Jun. 30, 2022	3,930,962	186	(930,284)	1,698,490	3,491,675	(329,105)
Balance, beginning of period at Mar. 31, 2022		193	(920,958)	1,871,763	3,326,119	(129,553)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		2		275
Shares repurchased under share repurchase program (Note 14)		(9)		(183,758)
Repurchases of common stock under incentive plans			(9,326)
Share-based compensation				10,210
Net income	201,193				201,193
Dividends and dividend equivalents declared					(35,637)
Net unrealized gains (losses) on investments, net of tax	(199,552)					(199,552)
Other adjustments to other comprehensive income (loss)						0
Balance, end of period at Jun. 30, 2022	3,930,962	186	(930,284)	1,698,490	3,491,675	(329,105)
Balance, beginning of period at Dec. 31, 2022	3,919,327	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)