SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Assets
Investments	$ 5,738,734	$ 5,424,688
Cash and cash equivalents	363,666	327,384
Accrued investment income	58,774	55,178
Total assets	6,538,380	6,213,793
Liabilities:
Senior notes	643,196	641,724
Convertible junior subordinated debentures	0	21,086
Other liabilities	160,009	154,966
Total liabilities	1,466,363	1,571,053
Shareholders’ equity:
Common stock	371,353	371,353
Paid-in capital	1,808,113	1,798,842
Treasury stock, at cost	(1,384,293)	(1,050,238)
Accumulated other comprehensive income, net of tax	(316,281)	(481,511)
Retained earnings	4,593,125	4,004,294
Total shareholders' equity	5,072,017	4,642,740	$ 4,861,382
Total liabilities and shareholders' equity	$ 6,538,380	$ 6,213,793
Parenthetical information [Abstract]
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (in shares)	371,353,000	371,353,000
Common stock, shares outstanding (in shares)	272,494,000	293,433,000
Treasury stock, shares at cost (in shares)	98,859,000	77,920,000
Revenues:
Investment income, net of expenses	$ 214,740	$ 167,476	156,438
Net gains (losses) on investments and other financial instruments	(14,141)	(7,463)	5,861
Total revenues	1,155,102	1,172,785	1,185,675
Expenses:
Interest expense	36,905	48,054	71,360
Loss on debt extinguishment	0	40,199	36,914
Total losses and expenses	252,873	82,751	383,898
Income before tax	902,229	1,090,034	801,777
Provision for income taxes	189,280	224,685	166,794
Net income	712,949	865,349	634,983
Other comprehensive income (loss), net of tax	165,230	(601,208)	(97,124)
Comprehensive income	878,179	264,141	537,859
Cash flows from operating activities:
Net income	712,949	865,349	634,983
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred federal	1,550	(5,235)	4,392
Loss on debt extinguishment	0	40,199	36,914
Change in certain assets and liabilities:
Accrued investment income	(3,596)	(3,276)	(1,905)
Net cash provided by operating activities	712,962	650,012	696,317
Cash flows from investing activities:
Purchases of investments	(1,469,540)	(674,406)	(1,531,129)
Proceeds from sales of investments	376,598	399,661	473,904
Net cash (used in) provided by investing activities	(179,190)	410,485	(160,749)
Cash flows from financing activities:
Conversion / purchase of convertible junior subordinated debentures	(28,637)	(89,118)	(98,610)
Redemption of 5.75% senior notes	0	(242,296)	0
Cash portion of loss on debt extinguishment	0	(39,514)	(36,914)
Repurchase of common stock	(337,182)	(385,573)	(290,818)
Dividends paid	(122,965)	(110,947)	(94,219)
Payment of withholding taxes related to share-based compensation net share settlement	(7,257)	(10,094)	(6,729)
Net cash used in financing activities	(496,041)	(1,032,542)	(527,290)
Net increase in cash and cash equivalents and restricted cash and cash equivalents	37,731	27,955	8,278
Cash and cash equivalents and restricted cash and cash equivalents at beginning of year	332,913	304,958	296,680
Cash and cash equivalents and restricted cash and cash equivalents at end of year	370,644	332,913	304,958
Fixed income
Assets
Fixed income, available-for-sale	5,601,540	5,342,667
Parenthetical information [Abstract]
Fixed income, amortized cost	5,939,483	5,859,688
Short-Term Investments
Assets
Fixed income, available-for-sale	121,573	67,031
Parenthetical information [Abstract]
Fixed income, amortized cost	121,539	67,097
Parent Company
Assets
Investments	709,333	407,509
Cash and cash equivalents	208,600	239,404
Investment in subsidiaries, at equity in net assets	4,777,121	4,502,261
Accounts receivable - affiliates	2,860	864
Income taxes - current and deferred	34,597	167,966
Accrued investment income	5,975	3,387
Total assets	5,738,486	5,321,391
Liabilities:
Senior notes	643,196	641,724
Convertible junior subordinated debentures	0	21,086
Accrued interest	12,797	13,271
Other liabilities	10,476	2,570
Total liabilities	666,469	678,651
Shareholders’ equity:
Common stock	371,353	371,353
Paid-in capital	1,808,113	1,798,842
Treasury stock, at cost	(1,384,293)	(1,050,238)
Accumulated other comprehensive income, net of tax	(316,281)	(481,511)
Retained earnings	4,593,125	4,004,294
Total shareholders' equity	5,072,017	4,642,740
Total liabilities and shareholders' equity	$ 5,738,486	$ 5,321,391
Parenthetical information [Abstract]
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (in shares)	371,353,000	371,353,000
Common stock, shares outstanding (in shares)	272,494,000	293,433,000
Treasury stock, shares at cost (in shares)	98,859,000	77,920,000
Revenues:
Investment income, net of expenses	$ 30,123	$ 7,193	3,850
Net gains (losses) on investments and other financial instruments	(47)	(2,628)	490
Total revenues	30,076	4,565	4,340
Expenses:
Operating expenses	1,555	1,575	1,644
Interest expense	36,905	47,601	68,359
Loss on debt extinguishment	0	38,870	36,914
Total losses and expenses	38,460	88,046	106,917
Income before tax	(8,384)	(83,481)	(102,577)
Provision for income taxes	(1,734)	(17,851)	(21,240)
Equity in net income of subsidiaries	719,599	930,979	716,320
Net income	712,949	865,349	634,983
Other comprehensive income (loss), net of tax	165,230	(601,208)	(97,124)
Comprehensive income	878,179	264,141	537,859
Cash flows from operating activities:
Net income	712,949	865,349	634,983
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries	(719,599)	(930,979)	(716,320)
Dividends received from subsidiaries	326,013	626,695	400,000
Deferred federal	86,219	119,588	(21,551)
Loss on debt extinguishment	0	38,870	36,914
Other	26,726	33,619	29,799
Change in certain assets and liabilities:
Accounts receivable - affiliates	(1,996)	1,266	(680)
Income taxes receivable	45,913	(43,123)	(306)
Accrued investment income	(499)	931	1,118
Accrued interest	349	(7,230)	(2,503)
Net cash provided by operating activities	476,075	704,986	361,454
Cash flows from investing activities:
Purchases of investments	(566,795)	(1,457)	(339,384)
Proceeds from sales of investments	555,957	287,924	556,384
Net cash (used in) provided by investing activities	(10,838)	286,467	217,000
Cash flows from financing activities:
Conversion / purchase of convertible junior subordinated debentures	(28,637)	(89,118)	(98,610)
Redemption of 5.75% senior notes	0	(242,296)	0
Cash portion of loss on debt extinguishment	0	(38,185)	(36,914)
Repurchase of common stock	(337,182)	(385,573)	(290,818)
Dividends paid	(122,965)	(110,947)	(94,219)
Payment of withholding taxes related to share-based compensation net share settlement	(7,257)	(10,094)	(6,729)
Net cash used in financing activities	(496,041)	(876,213)	(527,290)
Net increase in cash and cash equivalents and restricted cash and cash equivalents	(30,804)	115,240	51,164
Cash and cash equivalents and restricted cash and cash equivalents at beginning of year	239,404	124,164	73,000
Cash and cash equivalents and restricted cash and cash equivalents at end of year	208,600	239,404	$ 124,164
Parent Company | Fixed income
Assets
Fixed income, available-for-sale	598,420	287,332
Parenthetical information [Abstract]
Fixed income, amortized cost	597,944	299,649
Parent Company | Short-Term Investments
Assets
Fixed income, available-for-sale	110,913	120,177
Parenthetical information [Abstract]
Fixed income, amortized cost	$ 110,889	$ 120,102