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		2,944
Share-based compensation					20,996
Shares repurchased under share repurchase program (Note 14)			(11)		(256,967)
Repurchases of common stock under incentive plans				(10,240)
Net income	$ 407,226					407,226
Dividends and dividend equivalents declared						(78,865)
Net unrealized gains (losses) on investments, net of tax	(111,501)						(111,501)
Other adjustments to other comprehensive income (loss)							0
Balance, end of period at Sep. 30, 2021		$ 4,257,936	200	(920,355)	2,012,870	3,012,997	152,224
Balance, beginning of period at Jun. 30, 2021			207	(920,225)	2,161,857	2,913,138	178,856
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			0		1,007
Share-based compensation					8,282
Shares repurchased under share repurchase program (Note 14)			(7)		(158,276)
Repurchases of common stock under incentive plans				(130)
Net income	126,412					126,412
Dividends and dividend equivalents declared						(26,553)
Net unrealized gains (losses) on investments, net of tax	(26,632)						(26,632)
Other adjustments to other comprehensive income (loss)							0
Balance, end of period at Sep. 30, 2021		4,257,936	200	(920,355)	2,012,870	3,012,997	152,224
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		3,163
Share-based compensation					31,280
Shares repurchased under share repurchase program (Note 14)			(20)		(399,200)
Repurchases of common stock under incentive plans				(9,598)
Net income	580,604					580,604
Dividends and dividend equivalents declared						(104,669)
Net unrealized gains (losses) on investments, net of tax	(622,392)						(622,392)
Other adjustments to other comprehensive income (loss)							84
Balance, end of period at Sep. 30, 2022	3,738,050	3,738,050	176	(930,396)	1,513,615	3,656,870	(502,215)
Balance, beginning of period at Jun. 30, 2022			186	(930,284)	1,698,490	3,491,675	(329,105)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			0		1,525
Share-based compensation					7,714
Shares repurchased under share repurchase program (Note 14)			(10)		(194,114)
Repurchases of common stock under incentive plans				(112)
Net income	198,280					198,280
Dividends and dividend equivalents declared						(33,085)
Net unrealized gains (losses) on investments, net of tax	(173,110)						(173,110)
Other adjustments to other comprehensive income (loss)							0
Balance, end of period at Sep. 30, 2022	$ 3,738,050	$ 3,738,050	$ 176	$ (930,396)	$ 1,513,615	$ 3,656,870	$ (502,215)