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		3,951
Shares repurchased under share repurchase program (Note 14)		(3)		(70,444)
Repurchases of common stock under incentive plans			(14,861)
Share-based compensation				29,564
Net income	$ 460,426				460,426
Dividends and dividend equivalents declared					(110,780)
Net unrealized gains (losses) on investments, net of tax	(64,462)					(64,462)
Other adjustments to other comprehensive income (loss)						179
Balance, end of period at Sep. 30, 2023	4,152,899	175	(945,504)	1,482,712	4,136,598	(521,082)
Balance, beginning of period at Jun. 30, 2023		177	(945,032)	1,522,895	4,016,482	(423,686)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		0		1,094
Shares repurchased under share repurchase program (Note 14)		(2)		(50,436)
Repurchases of common stock under incentive plans			(472)
Share-based compensation				9,159
Net income	156,582				156,582
Dividends and dividend equivalents declared					(36,466)
Net unrealized gains (losses) on investments, net of tax	(97,396)					(97,396)
Other adjustments to other comprehensive income (loss)						0
Balance, end of period at Sep. 30, 2023	4,152,899	175	(945,504)	1,482,712	4,136,598	(521,082)
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		3,592
Shares repurchased under share repurchase program (Note 14)		(4)		(150,165)
Repurchases of common stock under incentive plans			(21,847)
Share-based compensation				31,025
Net income	456,149				456,149
Dividends and dividend equivalents declared					(115,455)
Net unrealized gains (losses) on investments, net of tax	97,819					97,819
Other adjustments to other comprehensive income (loss)						(68)
Balance, end of period at Sep. 30, 2024	4,698,853	171	(967,717)	1,315,046	4,584,453	(233,100)
Balance, beginning of period at Jun. 30, 2024		172	(967,218)	1,356,341	4,470,335	(377,311)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans		0		1,205
Shares repurchased under share repurchase program (Note 14)		(1)		(49,485)
Repurchases of common stock under incentive plans			(499)
Share-based compensation				6,985
Net income	151,892				151,892
Dividends and dividend equivalents declared					(37,774)
Net unrealized gains (losses) on investments, net of tax	144,211					144,211
Other adjustments to other comprehensive income (loss)						0
Balance, end of period at Sep. 30, 2024	$ 4,698,853	$ 171	$ (967,717)	$ 1,315,046	$ 4,584,453	$ (233,100)