SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Fixed income (amortized cost, 2016 – $247,396; 2015 – $385,281)												$ 4,685,222	$ 4,657,561
Cash and cash equivalents	$ 155,410			$ 181,120	$ 181,120			$ 197,882	$ 181,120	$ 197,882	$ 332,692	155,410	181,120	$ 197,882
Accrued investment income												44,073	40,224
Other assets												73,345	80,102
Total assets												5,734,529	5,868,343
Liabilities:
Senior notes												417,406	0
Convertible senior notes												349,461	822,301
Convertible junior subordinated debentures												256,872	389,522
Total liabilities												3,185,687	3,632,203
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2016 - 359,400; 2015 - 340,097; outstanding 2016 - 340,663; 2015 - 339,657)												359,400	340,097
Paid-in capital												1,782,337	1,670,238
Treasury stock (shares at cost 2016 – 18,737; 2015 – 440)												(150,359)	(3,362)
Accumulated other comprehensive loss, net of tax												(75,100)	(60,880)
Retained earnings												632,564	290,047
Total shareholders' equity												2,548,842	2,236,140	1,036,903
Total liabilities and shareholders' equity												5,734,529	5,868,343
Parenthetical information [Abstract]
Fixed income, amortized cost												$ 4,717,211	$ 4,684,148
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)												359,400,000	340,097,000
Common stock, shares outstanding (in shares)												340,663,000	339,657,000
Treasury stock, shares at cost (in shares)												18,737,000	440,000
Revenues:
Investment income, net of expenses	28,094	$ 27,515	$ 27,248	27,809	27,926	$ 25,939	$ 25,756	24,120	110,666	103,741	87,647
Realized (losses) gains	(52)	5,092	836	3,056	1,228	640	166	26,327	8,932	28,361	1,357
Total revenues									1,062,483	1,041,288	942,634
Expenses:
Interest expense									56,672	68,932	69,648
Loss on debt extinguishment	0	75,223	1,868	13,440	507	0	0	0	90,531	507	837
Total losses and expenses									547,769	553,601	687,911
Income before tax									514,714	487,687	254,723
Provision for (benefit from) income taxes	54,551	27,131	56,018	34,497	6,584	(695,604)	1,322	3,385	172,197	(684,313)	2,774
Net income	107,487	56,618	109,221	69,191	102,418	822,852	113,654	133,076	342,517	1,172,000	251,949
Other comprehensive (loss) income, net of tax									(14,220)	20,461	36,385
Comprehensive income									328,297	1,192,461	288,334
Cash flows from operating activities:
Net income	107,487	56,618	109,221	69,191	102,418	822,852	113,654	133,076	342,517	1,172,000	251,949
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Deferred Federal									160,657	(686,652)	1
Loss on debt extinguishment	0	$ 75,223	$ 1,868	13,440	507	$ 0	$ 0	0	90,531	507	837
Change in certain assets and liabilities:
Accrued investment income									(3,849)	(9,706)	1,142
Cash flows from investing activities:
Purchase of fixed income									(1,360,386)	(2,462,844)	(1,979,917)
Sale of fixed income									728,042	1,796,153	1,147,624
Cash flows from financing activities:
Proceeds from issuance of long-term debt									573,094	0	0
Repayment of long-term debt									0	(61,953)	(20,772)
Repurchase of convertible senior notes									(363,778)	(11,152)	0
Repurchase of common stock									(147,127)	0	0
Excess tax benefits related to share-based compensation									67	2,117	0
Net decrease in cash and cash equivalents									(25,710)	(16,762)	(134,810)
Cash and cash equivalents at beginning of year				181,120				197,882	181,120	197,882	332,692
Cash and cash equivalents at end of year	155,410				181,120				155,410	181,120	197,882
Parent Company
ASSETS
Fixed income (amortized cost, 2016 – $247,396; 2015 – $385,281)												$ 245,435	$ 382,565
Cash and cash equivalents	37,666			19,417	19,417			10,507	19,417	10,507	20,725	37,666	19,417	$ 10,507
Investment in subsidiaries, at equity in net assets												3,150,671	2,903,944
Accounts receivable - affiliates												780	938
Income taxes - current and deferred												289,703	151,318
Accrued investment income												1,749	3,700
Other assets												80	123
Total assets												3,726,084	3,462,005
Liabilities:
Senior notes												417,406	0
Convertible senior notes												349,461	822,301
Convertible junior subordinated debentures												389,522	389,522
Accrued interest												20,853	14,042
Total liabilities												1,177,242	1,225,865
Shareholders’ equity:
Common stock (one dollar par value, shares authorized 1,000,000; shares issued 2016 - 359,400; 2015 - 340,097; outstanding 2016 - 340,663; 2015 - 339,657)												359,400	340,097
Paid-in capital												1,782,337	1,670,238
Treasury stock (shares at cost 2016 – 18,737; 2015 – 440)												(150,359)	(3,362)
Accumulated other comprehensive loss, net of tax												(75,100)	(60,880)
Retained earnings												632,564	290,047
Total shareholders' equity												2,548,842	2,236,140
Total liabilities and shareholders' equity												3,726,084	3,462,005
Parenthetical information [Abstract]
Fixed income, amortized cost												$ 247,396	$ 385,281
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)												359,400,000	340,097,000
Common stock, shares outstanding (in shares)												340,663,000	339,657,000
Treasury stock, shares at cost (in shares)												18,737,000	440,000
Revenues:
Investment income, net of expenses									3,807	7,586	6,985
Realized (losses) gains									646	357	395
Total revenues									4,453	7,943	7,380
Expenses:
Operating expenses									1,409	582	546
Interest expense									64,598	68,932	69,648
Loss on debt extinguishment									82,234	507	837
Total losses and expenses									148,241	70,021	71,031
Income before tax									(143,788)	(62,078)	(63,651)
Provision for (benefit from) income taxes									(52,575)	(125,487)	0
Equity in net income of subsidiaries									433,730	1,108,591	315,600
Net income									342,517	1,172,000	251,949
Other comprehensive (loss) income, net of tax									(14,220)	20,461	36,385
Comprehensive income									328,297	1,192,461	288,334
Cash flows from operating activities:
Net income									342,517	1,172,000	251,949
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in net income of subsidiaries									(433,730)	(1,108,591)	(315,600)
Dividends received from subsidiaries									64,000	6,500	0
Deferred Federal									(55,988)	(125,532)	0
Loss on debt extinguishment									82,234	507	837
Other									11,625	22,342	14,025
Change in certain assets and liabilities:
Accounts receivable - affiliates									158	(626)	68
Income taxes receivable									3,602	(8,308)	480
Accrued investment income									1,951	(265)	194
Accrued interest									6,811	(652)	(188)
Net cash used in operating activities									23,180	(42,625)	(48,235)
Cash flows from investing activities:
Capital distributions from subsidiaries									51,987	32,000	0
Capital contributions to subsidiaries									(36,025)	0	0
Purchase of fixed income									(194,751)	(295,010)	(553,538)
Sale of fixed income									330,142	386,385	613,322
Net cash provided by investing activities									151,353	123,375	59,784
Cash flows from financing activities:
Proceeds from issuance of long-term debt									416,967	0	0
Repayment of long-term debt									0	(61,953)	(21,767)
Repurchase of convertible senior notes									(426,191)	(12,004)	0
Repurchase of common stock									(147,127)	0	0
Excess tax benefits related to share-based compensation									67	2,117	0
Net cash used in financing activities									(156,284)	(71,840)	(21,767)
Net decrease in cash and cash equivalents									18,249	8,910	(10,218)
Cash and cash equivalents at beginning of year				$ 19,417				$ 10,507	19,417	10,507	20,725
Cash and cash equivalents at end of year	$ 37,666				$ 19,417				$ 37,666	$ 19,417	$ 10,507