Condensed Consolidated Statements of Changes in Common Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Parent	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance, beginning of period at Dec. 31, 2020			$ 210	$ (910,115)	$ 2,245,897	$ 2,684,636	$ 263,725
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1		1,937
Share-based compensation					12,714
Shares repurchased under share repurchase program (Note 14)			(4)		(98,691)
Repurchases of common stock under incentive plans				(10,110)
Net income	$ 280,814					280,814
Dividends and dividend equivalents declared						(52,312)
Net unrealized gains (losses) on investments, net of tax	(84,869)						(84,869)
Other adjustments to other comprehensive income (loss)							0
Balance, end of period at Jun. 30, 2021		$ 4,333,833	207	(920,225)	2,161,857	2,913,138	178,856
Balance, beginning of period at Mar. 31, 2021			210	(910,347)	2,242,950	2,785,744	116,735
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1		770
Share-based compensation					8,191
Shares repurchased under share repurchase program (Note 14)			(4)		(90,054)
Repurchases of common stock under incentive plans				(9,878)
Net income	155,206					155,206
Dividends and dividend equivalents declared						(27,812)
Net unrealized gains (losses) on investments, net of tax	62,121						62,121
Other adjustments to other comprehensive income (loss)							0
Balance, end of period at Jun. 30, 2021		4,333,833	207	(920,225)	2,161,857	2,913,138	178,856
Balance, beginning of period at Dec. 31, 2021	4,258,796		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
Share-based compensation					23,566
Shares repurchased under share repurchase program (Note 14)			(10)		(205,086)
Repurchases of common stock under incentive plans				(9,486)
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	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
Share-based compensation					10,210
Shares repurchased under share repurchase program (Note 14)			(9)		(183,758)
Repurchases of common stock under incentive plans				(9,326)
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	$ 3,930,962	$ 186	$ (930,284)	$ 1,698,490	$ 3,491,675	$ (329,105)