SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Fixed income, available-for-sale, at fair value (amortized cost, 2019 – $287,489; 2018 – $203,743)	$ 5,737,892				$ 5,151,987				$ 5,737,892	$ 5,151,987
Cash and cash equivalents	161,847				151,892				161,847	151,892
Accrued investment income	49,705				48,001				49,705	48,001
Other assets	99,347				85,572				99,347	85,572
Total assets	6,229,571				5,677,802				6,229,571	5,677,802
Liabilities:
Senior notes	420,867				419,713				420,867	419,713
Convertible junior subordinated debentures	256,872				256,872				256,872	256,872
Other liabilities	151,962				180,322				151,962	180,322
Total liabilities	1,920,337				2,095,911				1,920,337	2,095,911
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2019 - 371,353; 2018 - 371,353; outstanding 2019 - 347,308; 2018 - 355,371)	371,353				371,353				371,353	371,353
Paid-in capital	1,869,719				1,862,536				1,869,719	1,862,536
Treasury stock (shares at cost 2019 – 24,045; 2018 – 15,982)	(283,196)				(175,059)				(283,196)	(175,059)
Accumulated other comprehensive income (loss), net of tax	72,708				(124,214)				72,708	(124,214)
Retained earnings	2,278,650				1,647,275				2,278,650	1,647,275
Total shareholders' equity	4,309,234				3,581,891				4,309,234	3,581,891	$ 3,154,526
Total liabilities and shareholders' equity	6,229,571				5,677,802				6,229,571	5,677,802
Parenthetical information [Abstract]
Fixed income, amortized cost	$ 5,562,550				$ 5,196,784				$ 5,562,550	$ 5,196,784
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)	347,308,000				355,371,000				347,308,000	355,371,000
Treasury stock, shares at cost (in shares)	24,045,000				15,982,000				24,045,000	15,982,000
Revenues:
Investment income, net of expenses	$ 41,322	$ 42,715	$ 42,423	$ 40,585	$ 38,328	$ 36,380	$ 34,502	$ 32,121	$ 167,045	$ 141,331	120,871
Realized (losses) gains	1,320	4,205	307	(526)	(241)	1,114	(1,897)	(329)	5,306	(1,353)	231
Total revenues									1,213,977	1,123,848	1,066,054
Expenses:
Interest expense									52,656	52,993	57,035
Loss on debt extinguishment									0	0	65
Total losses and expenses									366,000	279,698	281,558
Income before tax									847,977	844,150	784,496
Provision for income taxes	45,599	46,186	43,433	38,996	36,963	49,994	50,708	36,388	174,214	174,053	428,735
Net income	177,110	176,934	167,778	151,941	157,746	181,900	186,814	143,637	673,763	670,097	355,761
Other comprehensive income (loss), net of tax									196,922	(80,413)	41,739
Comprehensive income									870,685	589,684	397,500
Cash flows from operating activities:
Net income	177,110	$ 176,934	$ 167,778	151,941	157,746	$ 181,900	$ 186,814	143,637	673,763	670,097	355,761
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred Federal									11,860	185,598	351,677
Loss on debt extinguishment									0	0	65
Change in certain assets and liabilities:
Accrued investment income									(1,704)	(1,941)	(1,987)
Net cash provided by operating activities									609,532	544,517	406,657
Cash flows from investing activities:
Net cash used in investing activities									(422,108)	(317,780)	(303,641)
Cash flows from financing activities:
Proceeds from revolving credit facility									0	0	150,000
Repayment of revolving credit facility									0	0	(150,000)
Purchase or repayment of convertible senior notes									0	0	(145,620)
Repurchase of common stock									(125,766)	(163,419)	0
Dividends paid									(41,914)	0	0
Payment of debt issuance costs									0	0	(1,630)
Payment of withholding taxes related to share-based compensation net share settlement									(5,726)	(8,131)	(6,821)
Net cash used in financing activities									(173,406)	(171,550)	(158,575)
Net increase (decrease) in cash and cash equivalents and restricted cash and cash equivalents									14,018	55,187	(55,559)
Cash and cash equivalents and restricted cash and cash equivalents at beginning of year				155,038				99,851	155,038	99,851	155,410
Cash and cash equivalents and restricted cash and cash equivalents at end of year	169,056				155,038				169,056	155,038	99,851
Parent Company
Assets
Fixed income, available-for-sale, at fair value (amortized cost, 2019 – $287,489; 2018 – $203,743)	288,362				201,507				288,362	201,507
Cash and cash equivalents	36,621				46,502				36,621	46,502
Investment in subsidiaries, at equity in net assets	4,611,356				3,981,970				4,611,356	3,981,970
Accounts receivable - affiliates	2,129				1,396				2,129	1,396
Income taxes - current and deferred	196,978				186,561				196,978	186,561
Accrued investment income	2,498				2,020				2,498	2,020
Other assets	81				740				81	740
Total assets	5,138,025				4,420,696				5,138,025	4,420,696
Liabilities:
Senior notes	420,867				419,713				420,867	419,713
Convertible junior subordinated debentures	389,522				389,522				389,522	389,522
Accrued interest	17,928				17,930				17,928	17,930
Other liabilities	474				11,640				474	11,640
Total liabilities	828,791				838,805				828,791	838,805
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2019 - 371,353; 2018 - 371,353; outstanding 2019 - 347,308; 2018 - 355,371)	371,353				371,353				371,353	371,353
Paid-in capital	1,869,719				1,862,536				1,869,719	1,862,536
Treasury stock (shares at cost 2019 – 24,045; 2018 – 15,982)	(283,196)				(175,059)				(283,196)	(175,059)
Accumulated other comprehensive income (loss), net of tax	72,708				(124,214)				72,708	(124,214)
Retained earnings	2,278,650				1,647,275				2,278,650	1,647,275
Total shareholders' equity	4,309,234				3,581,891				4,309,234	3,581,891
Total liabilities and shareholders' equity	5,138,025				4,420,696				5,138,025	4,420,696
Parenthetical information [Abstract]
Fixed income, amortized cost	$ 287,489				$ 203,743				$ 287,489	$ 203,743
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)	347,308,000				355,371,000				347,308,000	355,371,000
Treasury stock, shares at cost (in shares)	24,045,000				15,982,000				24,045,000	15,982,000
Revenues:
Investment income, net of expenses									$ 7,695	$ 4,685	3,177
Realized (losses) gains									(311)	(532)	(13)
Total revenues									7,384	4,153	3,164
Expenses:
Operating expenses									793	637	642
Interest expense									61,593	61,930	65,972
Loss on debt extinguishment									0	0	65
Total losses and expenses									62,386	62,567	66,679
Income before tax									(55,002)	(58,414)	(63,515)
Provision for income taxes									(12,263)	(13,517)	95,517
Equity in net income of subsidiaries									716,502	714,994	514,793
Net income									673,763	670,097	355,761
Other comprehensive income (loss), net of tax									196,922	(80,413)	41,739
Comprehensive income									870,685	589,684	397,500
Cash flows from operating activities:
Net income									673,763	670,097	355,761
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries									(716,502)	(714,994)	(514,793)
Dividends received from subsidiaries									154,413	199,692	110,145
Deferred Federal									(10,416)	(11,756)	96,741
Loss on debt extinguishment									0	0	65
Other									21,104	24,303	18,716
Change in certain assets and liabilities:
Accounts receivable - affiliates									(735)	18	(634)
Income taxes receivable									1	17,859	297
Accrued investment income									(478)	112	(192)
Accrued interest									(2)	(4)	(2,819)
Net cash provided by operating activities									121,148	185,327	63,287
Cash flows from investing activities:
Purchases of investments									(117,663)	(83,003)	(97,091)
Proceeds from sales of investments									160,040	93,481	176,960
Net cash used in investing activities									42,377	10,478	79,869
Cash flows from financing activities:
Proceeds from revolving credit facility									0	0	150,000
Repayment of revolving credit facility									0	0	(150,000)
Purchase or repayment of convertible senior notes									0	0	(150,124)
Repurchase of common stock									(125,766)	(163,419)	0
Dividends paid									(41,914)	0	0
Payment of debt issuance costs									0	0	(1,630)
Payment of withholding taxes related to share-based compensation net share settlement									(5,726)	(8,131)	(6,821)
Net cash used in financing activities									(173,406)	(171,550)	(158,575)
Net increase (decrease) in cash and cash equivalents and restricted cash and cash equivalents									(9,881)	24,255	(15,419)
Cash and cash equivalents and restricted cash and cash equivalents at beginning of year				$ 46,502				$ 22,247	46,502	22,247	37,666
Cash and cash equivalents and restricted cash and cash equivalents at end of year	$ 36,621				$ 46,502				$ 36,621	$ 46,502	$ 22,247