SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed income (amortized cost, 2017 – $195,846; 2016 – $247,396)												$ 4,983,315	$ 4,685,222
Cash and cash equivalents	$ 99,851			$ 155,410	$ 155,410			$ 181,120	$ 155,410	$ 181,120	$ 197,882	99,851	155,410	$ 181,120
Accrued investment income												46,060	44,073
Other assets												78,478	73,345
Total assets												5,619,499	5,734,529
Liabilities:
Senior notes												418,560	417,406
Convertible senior notes												0	349,461
Convertible junior subordinated debentures												256,872	256,872
Total liabilities												2,464,973	3,185,687
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2017 - 370,567; 2016 - 359,400; outstanding 2017 - 370,567; 2016 - 340,663)												370,567	359,400
Paid-in capital												1,850,582	1,782,337
Treasury stock (shares at cost 2016 – 18,737)												0	(150,359)
Accumulated other comprehensive loss, net of tax												(43,783)	(75,100)
Retained earnings												977,160	632,564
Total shareholders' equity												3,154,526	2,548,842	2,236,140
Total liabilities and shareholders' equity												5,619,499	5,734,529
Parenthetical information [Abstract]
Fixed income, amortized cost												$ 4,946,278	$ 4,717,211
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)												370,567,000	359,400,000
Common stock, shares outstanding (in shares)												370,567,000	340,663,000
Treasury stock, shares at cost (in shares)												0	18,737,000
Revenues:
Investment income, net of expenses	31,276	$ 30,402	$ 29,716	29,477	28,094	$ 27,515	$ 27,248	27,809	120,871	110,666	103,741
Realized gains (losses)	460	(47)	(42)	(122)	(52)	5,092	836	3,056	249	8,932	28,361
Total revenues									1,066,054	1,062,483	1,041,288
Expenses:
Interest expense									57,035	56,672	68,932
Loss on debt extinguishment	0	0	65	0	0	75,223	1,868	13,440	65	90,531	507
Total losses and expenses									281,558	547,769	553,601
Income before tax									784,496	514,714	487,687
Provision for (benefit from) income taxes	218,142	64,440	61,994	84,159	54,551	27,131	56,018	34,497	428,735	172,197	(684,313)
Net income	27,314	120,027	118,622	89,798	107,487	56,618	109,221	69,191	355,761	342,517	1,172,000
Other comprehensive income (loss), net of tax									41,739	(14,220)	20,461
Comprehensive income									397,500	328,297	1,192,461
Cash flows from operating activities:
Net income	27,314	120,027	118,622	89,798	107,487	56,618	109,221	69,191	355,761	342,517	1,172,000
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Deferred Federal									351,677	160,657	(686,652)
Loss on debt extinguishment	0	$ 0	$ 65	0	0	$ 75,223	$ 1,868	13,440	65	90,531	507
Change in certain assets and liabilities:
Accrued investment income									(1,987)	(3,849)	(9,706)
Net cash provided by operating activities									406,657	224,760	161,395
Cash flows from investing activities:
Purchase of fixed income									(1,293,616)	(1,360,386)	(2,462,844)
Sale of fixed income									246,908	728,042	1,796,153
Net cash used in investing activities									(303,641)	(93,392)	(96,958)
Cash flows from financing activities:
Proceeds from revolving credit facility									150,000	0	0
Repayment of revolving credit facility									(150,000)	0	0
Proceeds from issuance of long-term debt									0	573,094	0
Repayment of long-term debt									0	0	(61,953)
Purchase or repayment of convertible senior notes									(145,620)	(363,778)	(11,152)
Repurchase of common stock									0	(147,127)	0
Payment of debt issuance costs									(1,630)	(1,127)	0
Payment of withholding taxes related to share-based compensation net share settlement									(6,821)	(5,030)	(7,242)
Net cash used in financing activities									(158,575)	(157,078)	(81,199)
Net decrease in cash and cash equivalents									(55,559)	(25,710)	(16,762)
Cash and cash equivalents at beginning of year				155,410				181,120	155,410	181,120	197,882
Cash and cash equivalents at end of year	99,851				155,410				99,851	155,410	181,120
Parent Company
Assets
Fixed income (amortized cost, 2017 – $195,846; 2016 – $247,396)												$ 194,061	$ 245,435
Cash and cash equivalents	22,247			37,666	37,666			19,417	37,666	19,417	10,507	22,247	37,666	$ 19,417
Investment in subsidiaries, at equity in net assets												3,567,034	3,150,671
Accounts receivable - affiliates												1,414	780
Income taxes - current and deferred												192,570	289,703
Accrued investment income												1,941	1,749
Other assets												1,275	80
Total assets												3,980,542	3,726,084
Liabilities:
Senior notes												418,560	417,406
Convertible senior notes												0	349,461
Convertible junior subordinated debentures												389,522	389,522
Accrued interest												17,934	20,853
Total liabilities												826,016	1,177,242
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2017 - 370,567; 2016 - 359,400; outstanding 2017 - 370,567; 2016 - 340,663)												370,567	359,400
Paid-in capital												1,850,582	1,782,337
Treasury stock (shares at cost 2016 – 18,737)												0	(150,359)
Accumulated other comprehensive loss, net of tax												(43,783)	(75,100)
Retained earnings												977,160	632,564
Total shareholders' equity												3,154,526	2,548,842
Total liabilities and shareholders' equity												3,980,542	3,726,084
Parenthetical information [Abstract]
Fixed income, amortized cost												$ 195,846	$ 247,396
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)												370,567,000	359,400,000
Common stock, shares outstanding (in shares)												370,567,000	340,663,000
Treasury stock, shares at cost (in shares)												0	18,737,000
Revenues:
Investment income, net of expenses									3,177	3,807	7,586
Realized gains (losses)									(13)	646	357
Total revenues									3,164	4,453	7,943
Expenses:
Operating expenses									642	1,409	582
Interest expense									65,972	64,598	68,932
Loss on debt extinguishment									65	82,234	507
Total losses and expenses									66,679	148,241	70,021
Income before tax									(63,515)	(143,788)	(62,078)
Provision for (benefit from) income taxes									95,517	(52,575)	(125,487)
Equity in net income of subsidiaries									514,793	433,730	1,108,591
Net income									355,761	342,517	1,172,000
Other comprehensive income (loss), net of tax									41,739	(14,220)	20,461
Comprehensive income									397,500	328,297	1,192,461
Cash flows from operating activities:
Net income									355,761	342,517	1,172,000
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in net income of subsidiaries									(514,793)	(433,730)	(1,108,591)
Dividends received from subsidiaries									110,145	64,000	6,500
Deferred Federal									96,741	(55,988)	(125,532)
Loss on debt extinguishment									65	82,234	507
Other									18,716	16,722	31,701
Change in certain assets and liabilities:
Accounts receivable - affiliates									(634)	158	(626)
Income taxes receivable									297	3,602	(8,308)
Accrued investment income									(192)	1,951	(265)
Accrued interest									(2,819)	6,811	(652)
Net cash provided by operating activities									63,287	28,277	(33,266)
Cash flows from investing activities:
Capital distributions from subsidiaries									0	51,987	32,000
Capital contributions to subsidiaries									0	(36,025)	0
Purchase of fixed income									(97,091)	(194,751)	(295,010)
Sale of fixed income									176,960	330,142	386,385
Net cash used in investing activities									79,869	151,353	123,375
Cash flows from financing activities:
Proceeds from revolving credit facility									150,000	0	0
Repayment of revolving credit facility									(150,000)	0	0
Proceeds from issuance of long-term debt									0	418,094	0
Repayment of long-term debt									0	0	(61,953)
Purchase or repayment of convertible senior notes									(150,124)	(426,191)	(12,004)
Repurchase of common stock									0	(147,127)	0
Payment of debt issuance costs									(1,630)	(1,127)	0
Payment of withholding taxes related to share-based compensation net share settlement									(6,821)	(5,030)	(7,242)
Net cash used in financing activities									(158,575)	(161,381)	(81,199)
Net decrease in cash and cash equivalents									(15,419)	18,249	8,910
Cash and cash equivalents at beginning of year				$ 37,666				$ 19,417	37,666	19,417	10,507
Cash and cash equivalents at end of year	$ 22,247				$ 37,666				$ 22,247	$ 37,666	$ 19,417