Condensed Financial Information of The Registrant (Details) - USD ($)		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	Dec. 31, 2014	Jun. 30, 2006
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 2,808,770,000	$ 3,110,853,000
Debt Issuance Cost													(20,405,000)	(22,218,000)
Long-term debt													2,788,365,000	3,088,635,000
Assets
Fixed maturity securities available-for-sale													38,150,820,000	32,093,625,000
Short-term investments													93,304,000	76,710,000
Cash and cash equivalents		$ 1,303,524,000			$ 1,200,718,000	$ 1,200,718,000			$ 1,525,275,000	$ 1,200,718,000	$ 1,525,275,000	$ 1,645,669,000	1,303,524,000	1,200,718,000	$ 1,525,275,000	$ 1,645,669,000
Other assets													767,088,000	755,338,000
Total assets													60,514,818,000	53,097,879,000
Liabilities and Stockholders’ Equity
Other liabilities													1,102,975,000	1,041,880,000
Total stockholders equity													9,569,535,000	7,093,082,000	6,135,381,000	7,023,452,000
Total liabilities and stockholders’ equity													60,514,818,000	53,097,879,000
CONSOLIDATED STATEMENTS OF INCOME
Total investment related gains (losses), net										167,880,000	94,195,000	(164,750,000)
Income Tax Expense (Benefit)										(679,366,000)	342,503,000	242,629,000
Net Income (Loss) Attributable to Parent		1,216,888,000	$ 227,591,000	$ 232,190,000	145,512,000	190,149,000	$ 198,719,000	$ 236,103,000	76,472,000	1,822,181,000	701,443,000	502,166,000
Other Comprehensive Income (Loss), Net of Tax										767,930,000	431,045,000	(948,845,000)
Total comprehensive income (loss)										2,590,111,000	1,132,488,000	(446,679,000)
Debt Disclosure [Abstract]
Debt Instrument, Face Amount																	$ 850,000,000.0
Cash flows from operating activities
Net Income (Loss) Attributable to Parent		1,216,888,000	$ 227,591,000	$ 232,190,000	145,512,000	190,149,000	$ 198,719,000	$ 236,103,000	76,472,000	1,822,181,000	701,443,000	502,166,000
Other, net										24,216,000	117,949,000	(49,564,000)
Net cash provided by operating activities										1,982,308,000	1,465,718,000	2,088,615,000
Cash flows from investing activities
Sales of fixed maturity securities available-for-sale										7,308,608,000	4,584,828,000	5,461,687,000
Payments to Acquire Businesses, Net of Cash Acquired										0	0	(145,235,000)
Payments for (Proceeds from) Short-term Investments										52,302,000	465,628,000	(470,002,000)
Payments for (Proceeds from) Other Investing Activities										(121,206,000)	(97,790,000)	91,960,000
Net cash used in investing activities										(1,607,573,000)	(2,825,726,000)	(1,431,741,000)
Cash flows from financing activities
Dividends to stockholders										(117,291,000)	(100,371,000)	(93,381,000)
Purchases of treasury stock										(43,508,000)	(122,916,000)	(384,519,000)
Excess tax benefits from share-based payment arrangement cash flow										0	162,000	2,963,000
Exercise of stock options, net										7,292,000	15,321,000	11,151,000
Maturities of Senior Debt										302,582,000	2,479,000	2,380,000
Proceeds from affiliated long-term debt issuance										0	(799,984,000)	0
Payments Of Debt Issuance Costs										0	(8,766,000)	(4,748,000)
Net cash used in financing activities										(324,622,000)	1,055,389,000	(708,282,000)
Change in cash and cash equivalents										102,806,000	(324,557,000)	(120,394,000)
Cash and cash equivalents, beginning of period					1,200,718,000				1,525,275,000	1,200,718,000	1,525,275,000	1,645,669,000
Cash and cash equivalents, end of period		1,303,524,000				1,200,718,000				1,303,524,000	1,200,718,000	1,525,275,000
Supplementary information:
Interest paid										173,471,000	156,727,000	148,124,000
Income taxes paid, net of refunds										37,098,000	61,085,000	41,577,000
Cash Acquired from Acquisition										0	0	69,823,000
Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													2,796,913,000	3,096,413,000
Debt Issuance Cost													(21,334,000)	(23,164,000)
Long-term debt	[1]												2,775,579,000	3,073,249,000
Assets
Fixed maturity securities available-for-sale													710,303,000	1,154,559,000
Short-term investments													54,517,000	245,478,000
Cash and cash equivalents		15,176,000			43,718,000	43,718,000			39,452,000	43,718,000	39,452,000	48,819,000	15,176,000	43,718,000	$ 39,452,000	$ 48,819,000
Investments In Subsidiary													12,043,911,000	9,209,700,000
Loans and Leases Receivable, Related Parties													1,010,000,000	1,050,000,000
Other assets													234,707,000	258,379,000
Total assets													14,068,614,000	11,961,834,000
Liabilities and Stockholders’ Equity
Long-term debt - affiliated	[2]												500,000,000	500,000,000
Other liabilities													1,223,500,000	1,295,503,000
Total stockholders equity													9,569,535,000	7,093,082,000
Total liabilities and stockholders’ equity													14,068,614,000	11,961,834,000
CONSOLIDATED STATEMENTS OF INCOME
Interest and Dividend Income, Operating	[3]									131,067,000	602,830,000	321,645,000
Total investment related gains (losses), net										(5,187,000)	203,000	(324,000)
Operating Expenses										(20,517,000)	(20,742,000)	(13,652,000)
Interest and Debt Expense										(177,417,000)	(168,924,000)	(176,364,000)
Income (loss) before income tax and undistributed earnings of subsidiaries										(72,054,000)	413,367,000	131,305,000
Income Tax Expense (Benefit)										65,882,000	(23,911,000)	(19,465,000)
Net income (loss) before undistributed earnings of subsidiaries										(137,936,000)	437,278,000	150,770,000
Equity In Undistributed Earnings Of Subsidiaries										1,960,117,000	264,165,000	351,396,000
Net Income (Loss) Attributable to Parent										1,822,181,000	701,443,000	502,166,000
Other Comprehensive Income (Loss), Net of Tax										(7,672,000)	5,531,000	44,073,000
Total comprehensive income (loss)										1,814,509,000	706,974,000	546,239,000
Debt Disclosure [Abstract]
Proceeds from Dividends Received										0	478,602,000	196,445,000
Long-term Debt													500,000,000	500,000,000
Cash flows from operating activities
Net Income (Loss) Attributable to Parent										1,822,181,000	701,443,000	502,166,000
Equity Net Earnings Subsidiaries										(1,960,117,000)	(264,165,000)	(351,396,000)
Other, net										57,677,000	(63,795,000)	486,159,000
Net cash provided by operating activities										(80,259,000)	373,483,000	636,929,000
Cash flows from investing activities
Sales of fixed maturity securities available-for-sale										514,508,000	228,383,000	100,734,000
Purchases of fixed maturity securities available-for-sale										(75,000,000)	(984,397,000)	(52,698,000)
Purchases Of Subsidiary Debt Securities										40,000,000	20,000,000	(10,000,000)
Payments to Acquire Businesses, Net of Cash Acquired										0	0	(3,701,000)
Payments for (Proceeds from) Short-term Investments										0	102,508,000	(102,508,000)
Payments for (Proceeds from) Other Investing Activities										125,506,000	(109,914,000)	(7,542,000)
Capital Contributions To Subsidiaries										(62,500,000)	(314,142,000)	(103,832,000)
Net cash used in investing activities										542,514,000	(1,057,562,000)	(179,547,000)
Cash flows from financing activities
Dividends to stockholders										(117,291,000)	(100,371,000)	(93,381,000)
Purchases of treasury stock										(43,508,000)	(122,916,000)	(384,519,000)
Exercise of stock options, net										7,292,000	15,321,000	11,151,000
Net Change In Cash Collateral For Loaned Securities										(37,290,000)	105,093,000	0
Maturities of Senior Debt										(300,000,000)	0	0
Proceeds from affiliated long-term debt issuance										0	(799,984,000)	0
Payments Of Debt Issuance Costs										0	(8,766,000)	0
Net cash used in financing activities										(490,797,000)	688,345,000	(466,749,000)
Change in cash and cash equivalents										(28,542,000)	4,266,000	(9,367,000)
Cash and cash equivalents, beginning of period					$ 43,718,000				$ 39,452,000	43,718,000	39,452,000	48,819,000
Cash and cash equivalents, end of period		$ 15,176,000				$ 43,718,000				15,176,000	43,718,000	39,452,000
Supplementary information:
Interest paid										185,554,000	169,860,000	165,775,000
Income taxes paid, net of refunds										8,248,000	1,500,000	(120,680,000)
Cash Acquired from Acquisition										0	$ 0	$ 529,000
Senior Notes Due 2017 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													0	299,945,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 300,000,000
Debt Instrument, Interest Rate, Stated Percentage													5.625%
Senior Notes Due 2017 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 0	299,945,000
Debt Disclosure [Abstract]
Repayments of Long-term Debt										300,000,000
Senior Notes Due 2019 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													399,873,000	399,805,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage													6.45%
Senior Notes Due 2019 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 399,873,000	399,805,000
Debt Disclosure [Abstract]
Repayments of Long-term Debt										$ 400,000,000
Senior Notes Due 2021 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													399,245,000	399,025,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage													5.00%
Senior Notes Due 2021 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 399,245,000	399,025,000
Senior Notes Due 2023 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													399,138,000	398,986,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage													4.70%
Senior Notes Due 2023 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 399,138,000	398,986,000
Senior Notes Due 2026 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													399,987,000	399,985,000
Debt Issuance Cost													(8,800,000)
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage													3.95%
Senior Notes Due 2026 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 399,987,000	399,985,000
Subordinated Debentures Due 2042 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													400,000,000	400,000,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage													6.20%
Subordinated Debentures Due 2042 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 400,000,000	400,000,000
Subordinated Debentures Due 2056 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													400,000,000	400,000,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 400,000,000.0
Debt Instrument, Interest Rate, Stated Percentage													5.75%
Subordinated Debentures Due 2056 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 400,000,000	400,000,000
Junior Subordinated Debentures Due 2065 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													318,740,000	318,737,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													400,000,000
Junior Subordinated Debentures Due 2065 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 398,670,000	$ 398,667,000

[1] Long-term debt - unaffiliated consists of the following: 2017 2016$300 million 5.625% Senior Notes due 2017 $— $299,945$400 million 6.45% Senior Notes due 2019 399,873 399,805$400 million 5.00% Senior Notes due 2021 399,245 399,025$400 million 4.70% Senior Notes due 2023 399,138 398,986$400 million 3.95% Senior Notes due 2026 399,987 399,985$400 million 6.20% Subordinated Debentures due 2042 400,000 400,000$400 million 5.75% Subordinated Debentures due 2056 400,000 400,000$400 million Variable Rate Junior Subordinated Debentures due 2065 398,670 398,667Subtotal 2,796,913 3,096,413Unamortized debt issue costs (21,334) (23,164)Total $2,775,579 $3,073,249Repayments of long-term debt—unaffiliated due over the next five years total $400,000 in 2019 and $400,000 in 2021.

[2] Long-term debt—affiliated in 2017 and 2016 and consists of $500,000 of subordinated debt issued to various operating subsidiaries.

[3] Interest/Dividend income includes $478,602 and $196,445 of cash dividends received from consolidated subsidiaries in 2016 and 2015, respectively. In 2017 there were no cash dividends received from consolidated subsidiaries.