SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets
Fixed income, available-for-sale, at fair value (amortized cost, 2020 – $773,881; 2019 – $287,489)	$ 6,661,596				$ 5,737,892				$ 6,661,596	$ 5,737,892
Cash and cash equivalents	287,953				161,847				287,953	161,847
Accrued investment income	49,997				49,705				49,997	49,705
Other assets	104,478				105,089				104,478	105,089
Total assets	7,354,526				6,229,571				7,354,526	6,229,571
Liabilities:
Senior notes	879,379				420,867				879,379	420,867
Convertible junior subordinated debentures	208,814				256,872				208,814	256,872
Other liabilities	244,711				151,962				244,711	151,962
Total liabilities	2,655,540				1,920,337				2,655,540	1,920,337
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2020 - 371,353; 2019 - 371,353; outstanding 2020 - 338,573; 2019 - 347,308)	371,353				371,353				371,353	371,353
Paid-in capital	1,862,042				1,869,719				1,862,042	1,869,719
Treasury stock (shares at cost 2020 – 32,779; 2019 – 24,045)	(393,326)				(283,196)				(393,326)	(283,196)
Accumulated other comprehensive income, net of tax	216,821				72,708				216,821	72,708
Retained earnings	2,642,096				2,278,650				2,642,096	2,278,650
Total shareholders' equity	4,698,986				4,309,234				4,698,986	4,309,234	$ 3,581,891
Total liabilities and shareholders' equity	7,354,526				6,229,571				7,354,526	6,229,571
Parenthetical information [Abstract]
Fixed income, amortized cost	$ 6,317,164				$ 5,562,550				$ 6,317,164	$ 5,562,550
Common stock, par value (in dollars per share)	$ 1				$ 1				$ 1	$ 1
Common stock, shares authorized (in shares)	1,000,000,000				1,000,000,000				1,000,000,000	1,000,000,000
Common stock, shares issued (in shares)	371,353,000				371,353,000				371,353,000	371,353,000
Common stock, shares outstanding (in shares)	338,573,000				347,308,000				338,573,000	347,308,000
Treasury stock, shares at cost (in shares)	32,779,000				24,045,000				32,779,000	24,045,000
Revenues:
Investment income, net of expenses	$ 36,118	$ 37,252	$ 39,679	$ 41,347	$ 41,322	$ 42,715	$ 42,423	$ 40,585	$ 154,396	$ 167,045	141,331
Realized gains (losses)	2,901	2,259	6,701	1,891	1,320	4,205	307	(526)	13,752	5,306	(1,353)
Total revenues									1,199,146	1,213,977	1,123,848
Expenses:
Interest expense									59,595	52,656	52,993
Loss on debt extinguishment	0	26,736	0	0					26,736	0	0
Total losses and expenses									639,883	366,000	279,698
Income before tax									559,263	847,977	844,150
Provision for income taxes	38,782	33,518	2,436	38,434	45,599	46,186	43,433	38,996	113,170	174,214	174,053
Net income	151,430	130,811	14,047	149,805	177,110	176,934	167,778	151,941	446,093	673,763	670,097
Other comprehensive income (loss), net of tax									144,113	196,922	(80,413)
Comprehensive income									590,206	870,685	589,684
Cash flows from operating activities:
Net income	151,430	130,811	14,047	149,805	177,110	$ 176,934	$ 167,778	151,941	446,093	673,763	670,097
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred Federal									28,244	11,860	185,598
Loss on debt extinguishment	0	$ 26,736	$ 0	0					26,736	0	0
Change in certain assets and liabilities:
Accrued investment income									(292)	(1,704)	(1,941)
Net cash provided by operating activities									732,309	609,532	544,517
Cash flows from investing activities:
Purchases of investments									(2,636,972)	(1,394,126)	(1,459,473)
Proceeds from sales of investments									836,851	229,796	370,449
Net cash used in investing activities									(772,506)	(422,108)	(317,780)
Cash flows from financing activities:
Proceeds from issuance of senior notes									640,250	0	0
Purchase of senior notes									(179,735)	0	0
Payment of original issue discount - senior notes									(2,969)	0	0
Purchase of convertible junior subordinated debentures									(36,392)	0	0
Payment of original issue discount- convertible junior subordinated debentures									(15,049)	0	0
Cash portion of loss on debt extinguishment									(25,266)	0	0
Repurchase of common stock									(119,997)	(125,766)	(163,419)
Dividends paid									(82,061)	(41,914)	0
Payment of debt issuance costs									(2,020)	0	0
Payment of withholding taxes related to share-based compensation net share settlement									(8,940)	(5,726)	(8,131)
Net cash provided by (used in) financing activities									167,821	(173,406)	(171,550)
Net increase in cash and cash equivalents and restricted cash and cash equivalents									127,624	14,018	55,187
Cash and cash equivalents and restricted cash and cash equivalents at beginning of year				169,056				155,038	169,056	155,038	99,851
Cash and cash equivalents and restricted cash and cash equivalents at end of year	296,680				169,056				296,680	169,056	155,038
Parent Company
Assets
Fixed income, available-for-sale, at fair value (amortized cost, 2020 – $773,881; 2019 – $287,489)	773,995				288,362				773,995	288,362
Cash and cash equivalents	73,000				36,621				73,000	36,621
Investment in subsidiaries, at equity in net assets	4,740,074				4,611,356				4,740,074	4,611,356
Accounts receivable - affiliates	1,450				2,129				1,450	2,129
Income taxes -current and deferred	219,059				196,978				219,059	196,978
Accrued investment income	3,760				2,498				3,760	2,498
Other assets	0				81				0	81
Total assets	5,811,338				5,138,025				5,811,338	5,138,025
Liabilities:
Senior notes	879,379				420,867				879,379	420,867
Convertible junior subordinated debentures	208,814				389,522				208,814	389,522
Accrued interest	23,004				17,928				23,004	17,928
Other liabilities	1,155				474				1,155	474
Total liabilities	1,112,352				828,791				1,112,352	828,791
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2020 - 371,353; 2019 - 371,353; outstanding 2020 - 338,573; 2019 - 347,308)	371,353				371,353				371,353	371,353
Paid-in capital	1,862,042				1,869,719				1,862,042	1,869,719
Treasury stock (shares at cost 2020 – 32,779; 2019 – 24,045)	(393,326)				(283,196)				(393,326)	(283,196)
Accumulated other comprehensive income, net of tax	216,821				72,708				216,821	72,708
Retained earnings	2,642,096				2,278,650				2,642,096	2,278,650
Total shareholders' equity	4,698,986				4,309,234				4,698,986	4,309,234
Total liabilities and shareholders' equity	5,811,338				5,138,025				5,811,338	5,138,025
Parenthetical information [Abstract]
Fixed income, amortized cost	$ 773,881				$ 287,489				$ 773,881	$ 287,489
Common stock, par value (in dollars per share)	$ 1				$ 1				$ 1	$ 1
Common stock, shares authorized (in shares)	1,000,000,000				1,000,000,000				1,000,000,000	1,000,000,000
Common stock, shares issued (in shares)	371,353,000				371,353,000				371,353,000	371,353,000
Common stock, shares outstanding (in shares)	338,573,000				347,308,000				338,573,000	347,308,000
Treasury stock, shares at cost (in shares)	32,779,000				24,045,000				32,779,000	24,045,000
Revenues:
Investment income, net of expenses									$ 7,090	$ 7,695	4,685
Realized gains (losses)									1,454	(311)	(532)
Total revenues									8,544	7,384	4,153
Expenses:
Operating expenses									719	793	637
Interest expense									65,472	61,593	61,930
Loss on debt extinguishment									35,033	0	0
Total losses and expenses									101,224	62,386	62,567
Income before tax									(92,680)	(55,002)	(58,414)
Provision for income taxes									(18,431)	(12,263)	(13,517)
Equity in net income of subsidiaries									520,342	716,502	714,994
Net income									446,093	673,763	670,097
Other comprehensive income (loss), net of tax									144,113	196,922	(80,413)
Comprehensive income									590,206	870,685	589,684
Cash flows from operating activities:
Net income									446,093	673,763	670,097
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries									(520,342)	(716,502)	(714,994)
Dividends received from subsidiaries									221,024	154,413	199,692
Deferred Federal									(18,252)	(10,416)	(11,756)
Loss on debt extinguishment									35,033	0	0
Other									19,088	21,104	24,303
Change in certain assets and liabilities:
Accounts receivable - affiliates									972	(735)	18
Income taxes receivable									0	1	17,859
Accrued investment income									(1,262)	(478)	112
Accrued interest									5,076	(2)	(4)
Net cash provided by operating activities									187,430	121,148	185,327
Cash flows from investing activities:
Purchases of investments									(1,131,060)	(117,663)	(83,003)
Proceeds from sales of investments									812,188	160,040	93,481
Net cash used in investing activities									(318,872)	42,377	10,478
Cash flows from financing activities:
Proceeds from issuance of senior notes									640,250	0	0
Purchase of senior notes									(179,735)	0	0
Payment of original issue discount - senior notes									(2,969)	0	0
Purchase of convertible junior subordinated debentures									(36,392)	0	0
Payment of original issue discount- convertible junior subordinated debentures									(15,049)	0	0
Cash portion of loss on debt extinguishment									(25,266)	0	0
Repurchase of common stock									(119,997)	(125,766)	(163,419)
Dividends paid									(82,061)	(41,914)	0
Payment of debt issuance costs									(2,020)	0	0
Payment of withholding taxes related to share-based compensation net share settlement									(8,940)	(5,726)	(8,131)
Net cash provided by (used in) financing activities									167,821	(173,406)	(171,550)
Net increase in cash and cash equivalents and restricted cash and cash equivalents									36,379	(9,881)	24,255
Cash and cash equivalents and restricted cash and cash equivalents at beginning of year				$ 36,621				$ 46,502	36,621	46,502	22,247
Cash and cash equivalents and restricted cash and cash equivalents at end of year	$ 73,000				$ 36,621				$ 73,000	$ 36,621	$ 46,502