SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Assets
Fixed income, available-for-sale	$ 6,587,581	$ 6,661,596
Cash and cash equivalents	284,690	287,953
Accrued investment income	51,902	49,997
Total assets	7,325,008	7,354,526
Liabilities:
Senior notes	881,508	879,379
Convertible junior subordinated debentures	110,204	208,814
Other liabilities	191,702	244,711
Total liabilities	2,463,626	2,655,540
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2021 - 371,353; 2020 - 371,353; shares outstanding 2021 - 320,336; 2020 - 338,573)	371,353	371,353
Paid-in capital	1,794,906	1,862,042
Treasury stock at cost (shares 2021 - 51,017; 2020 - 32,779)	(675,265)	(393,326)
Accumulated other comprehensive income, net of tax	119,697	216,821
Retained earnings	3,250,691	2,642,096
Total shareholders' equity	4,861,382	4,698,986	$ 4,309,234
Total liabilities and shareholders' equity	7,325,008	7,354,526
Parenthetical information [Abstract]
Fixed income, amortized cost	$ 6,397,658	$ 6,317,164
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)	320,336,000	338,573,000
Treasury stock, shares at cost (in shares)	51,017,000	32,779,000
Revenues:
Investment income, net of expenses	$ 156,438	$ 154,396	167,045
Net realized investment gains (losses)	6,582	13,752	5,306
Total revenues	1,185,675	1,199,146	1,213,977
Expenses:
Interest expense	71,360	59,595	52,656
Loss on debt extinguishment	36,914	26,736	0
Total losses and expenses	383,898	639,883	366,000
Income before tax	801,777	559,263	847,977
Provision for income taxes	166,794	113,170	174,214
Net income	634,983	446,093	673,763
Other comprehensive income (loss), net of tax	(97,124)	144,113	196,922
Comprehensive income	537,859	590,206	870,685
Cash flows from operating activities:
Net income	634,983	446,093	673,763
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred Federal	4,392	28,244	11,860
Loss on debt extinguishment	36,914	26,736	0
Change in certain assets and liabilities:
Accrued investment income	(1,905)	(292)	(1,704)
Net cash provided by operating activities	696,317	732,309	609,532
Cash flows from investing activities:
Purchases of investments	(1,531,129)	(2,636,972)	(1,394,126)
Proceeds from sales of investments	473,904	836,851	229,796
Net cash used in investing activities	(160,749)	(772,506)	(422,108)
Cash flows from financing activities:
Proceeds from issuance of senior notes	0	640,250	0
Purchase of senior notes	0	(179,735)	0
Payment of original issue discount - senior notes	0	(2,969)	0
Purchase of convertible junior subordinated debentures	(98,610)	(36,392)	0
Payment of original issue discount- convertible junior subordinated debentures	0	(15,049)	0
Cash portion of loss on debt extinguishment	(36,914)	(25,266)	0
Repurchase of common stock	(290,818)	(119,997)	(125,766)
Dividends paid	(94,219)	(82,061)	(41,914)
Payment of debt issuance costs	0	(2,020)	0
Payment of withholding taxes related to share-based compensation net share settlement	(6,729)	(8,940)	(5,726)
Net cash provided by (used in) financing activities	(527,290)	167,821	(173,406)
Net increase (decrease) in cash and cash equivalents and restricted cash and cash equivalents	8,278	127,624	14,018
Cash and cash equivalents and restricted cash and cash equivalents at beginning of year	296,680	169,056	155,038
Cash and cash equivalents and restricted cash and cash equivalents at end of year	304,958	296,680	169,056
Parent Company
Assets
Fixed income, available-for-sale	538,872	773,995
Cash and cash equivalents	124,164	73,000
Investment in subsidiaries, at equity in net assets	4,964,954	4,740,074
Accounts receivable - affiliates	2,130	1,450
Income taxes - current and deferred	242,427	219,059
Accrued investment income	2,642	3,760
Total assets	5,875,189	5,811,338
Liabilities:
Senior notes	881,508	879,379
Convertible junior subordinated debentures	110,204	208,814
Accrued interest	20,501	23,004
Other liabilities	1,594	1,155
Total liabilities	1,013,807	1,112,352
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2021 - 371,353; 2020 - 371,353; shares outstanding 2021 - 320,336; 2020 - 338,573)	371,353	371,353
Paid-in capital	1,794,906	1,862,042
Treasury stock at cost (shares 2021 - 51,017; 2020 - 32,779)	(675,265)	(393,326)
Accumulated other comprehensive income, net of tax	119,697	216,821
Retained earnings	3,250,691	2,642,096
Total shareholders' equity	4,861,382	4,698,986
Total liabilities and shareholders' equity	5,875,189	5,811,338
Parenthetical information [Abstract]
Fixed income, amortized cost	$ 550,324	$ 773,881
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)	320,336,000	338,573,000
Treasury stock, shares at cost (in shares)	51,017,000	32,779,000
Revenues:
Investment income, net of expenses	$ 3,850	$ 7,090	7,695
Net realized investment gains (losses)	490	1,454	(311)
Total revenues	4,340	8,544	7,384
Expenses:
Operating expenses	1,644	719	793
Interest expense	68,359	65,472	61,593
Loss on debt extinguishment	36,914	35,033	0
Total losses and expenses	106,917	101,224	62,386
Income before tax	(102,577)	(92,680)	(55,002)
Provision for income taxes	(21,240)	(18,431)	(12,263)
Equity in net income of subsidiaries	716,320	520,342	716,502
Net income	634,983	446,093	673,763
Other comprehensive income (loss), net of tax	(97,124)	144,113	196,922
Comprehensive income	537,859	590,206	870,685
Cash flows from operating activities:
Net income	634,983	446,093	673,763
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries	(716,320)	(520,342)	(716,502)
Dividends received from subsidiaries	400,000	221,024	154,413
Deferred Federal	(21,551)	(18,252)	(10,416)
Loss on debt extinguishment	36,914	35,033	0
Other	29,799	19,088	21,104
Change in certain assets and liabilities:
Accounts receivable - affiliates	(680)	972	(735)
Income taxes receivable	(306)	0	1
Accrued investment income	1,118	(1,262)	(478)
Accrued interest	(2,503)	5,076	(2)
Net cash provided by operating activities	361,454	187,430	121,148
Cash flows from investing activities:
Purchases of investments	(339,384)	(1,131,060)	(117,663)
Proceeds from sales of investments	556,384	812,188	160,040
Net cash used in investing activities	217,000	(318,872)	42,377
Cash flows from financing activities:
Proceeds from issuance of senior notes	0	640,250	0
Purchase of senior notes	0	(179,735)	0
Payment of original issue discount - senior notes	0	(2,969)	0
Purchase of convertible junior subordinated debentures	(98,610)	(36,392)	0
Payment of original issue discount- convertible junior subordinated debentures	0	(15,049)	0
Cash portion of loss on debt extinguishment	(36,914)	(25,266)	0
Repurchase of common stock	(290,818)	(119,997)	(125,766)
Dividends paid	(94,219)	(82,061)	(41,914)
Payment of debt issuance costs	0	(2,020)	0
Payment of withholding taxes related to share-based compensation net share settlement	(6,729)	(8,940)	(5,726)
Net cash provided by (used in) financing activities	(527,290)	167,821	(173,406)
Net increase (decrease) in cash and cash equivalents and restricted cash and cash equivalents	51,164	36,379	(9,881)
Cash and cash equivalents and restricted cash and cash equivalents at beginning of year	73,000	36,621	46,502
Cash and cash equivalents and restricted cash and cash equivalents at end of year	$ 124,164	$ 73,000	$ 36,621