Condensed Financial Information of The Registrant (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2006
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 2,806,524,000	$ 2,808,770,000
Debt Issuance Cost													(18,651,000)	(20,405,000)
Long-term debt													2,787,873,000	2,788,365,000
Assets
Fixed maturity securities available-for-sale													39,992,346,000	38,150,820,000
Short-term investments													142,598,000	93,304,000
Cash and cash equivalents		$ 1,889,733,000			$ 1,303,524,000	$ 1,303,524,000			$ 1,200,718,000	$ 1,303,524,000	$ 1,200,718,000	$ 1,525,275,000	1,889,733,000	1,303,524,000	$ 1,200,718,000	$ 1,525,275,000
Other assets													839,222,000	767,088,000
Total assets													64,535,245,000	60,514,818,000
Liabilities and Stockholders’ Equity
Other liabilities													1,396,200,000	1,102,975,000
Total stockholders equity													8,450,553,000	9,569,535,000	7,093,082,000	6,135,381,000
Total liabilities and stockholders’ equity													64,535,245,000	60,514,818,000
CONSOLIDATED STATEMENTS OF INCOME
Total investment related gains (losses), net										(170,088,000)	167,880,000	94,195,000
Income Tax Expense (Benefit)										129,978,000	(679,366,000)	342,503,000
Net Income (Loss) Attributable to Parent		110,039,000	$ 301,199,000	$ 204,374,000	100,230,000	1,216,888,000	$ 227,591,000	$ 232,190,000	145,512,000	715,842,000	1,822,181,000	701,443,000
Other Comprehensive Income (Loss), Net of Tax										(1,424,295,000)	767,930,000	431,045,000
Total comprehensive income (loss)										(708,453,000)	2,590,111,000	1,132,488,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount																	$ 850,000,000.0
Cash flows from operating activities
Net Income (Loss) Attributable to Parent		110,039,000	$ 301,199,000	$ 204,374,000	100,230,000	1,216,888,000	$ 227,591,000	$ 232,190,000	145,512,000	715,842,000	1,822,181,000	701,443,000
Other, net										(1,268,000)	24,216,000	117,949,000
Net cash provided by operating activities										1,581,121,000	1,982,308,000	1,465,718,000
Cash flows from investing activities
Sales of fixed maturity securities available-for-sale										9,339,632,000	7,308,608,000	4,584,828,000
Payments to Acquire Businesses, Net of Cash Acquired										(31,441,000)	0	0
Payments for (Proceeds from) Short-term Investments										129,329,000	52,302,000	465,628,000
Payments for (Proceeds from) Other Investing Activities										(365,242,000)	(121,206,000)	(97,790,000)
Net cash used in investing activities										(636,552,000)	(1,607,573,000)	(2,825,726,000)
Cash flows from financing activities
Dividends to stockholders										(140,110,000)	(117,291,000)	(100,371,000)
Purchases of treasury stock										(299,679,000)	(43,508,000)	(122,916,000)
Excess tax benefits from share-based payment arrangement cash flow										0	0	162,000
Exercise of stock options, net										3,459,000	7,292,000	15,321,000
Maturities of Senior Debt										2,690,000	302,582,000	2,479,000
Proceeds from affiliated long-term debt issuance										0	0	(799,984,000)
Payments Of Debt Issuance Costs										0	0	(8,766,000)
Net cash used in financing activities										(321,973,000)	(324,622,000)	1,055,389,000
Change in cash and cash equivalents										586,209,000	102,806,000	(324,557,000)
Cash and cash equivalents, beginning of period					1,303,524,000				1,200,718,000	1,303,524,000	1,200,718,000	1,525,275,000
Cash and cash equivalents, end of period		1,889,733,000				1,303,524,000				1,889,733,000	1,303,524,000	1,200,718,000
Supplementary information:
Interest paid										170,191,000	173,471,000	156,727,000
Income taxes paid, net of refunds										141,703,000	37,098,000	61,085,000
Cash Acquired from Acquisition										4,988,000	0	0
Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													2,797,357,000	2,796,913,000
Debt Issuance Cost													(19,601,000)	(21,334,000)
Long-term debt	[1]												2,777,756,000	2,775,579,000
Assets
Fixed maturity securities available-for-sale													595,303,000	710,303,000
Short-term investments													42,906,000	54,517,000
Cash and cash equivalents		20,641,000			15,176,000	15,176,000			43,718,000	15,176,000	43,718,000	39,452,000	20,641,000	15,176,000	$ 43,718,000	$ 39,452,000
Investments In Subsidiary													11,033,195,000	12,043,911,000
Loans and Leases Receivable, Related Parties													1,010,000,000	1,010,000,000
Other assets													239,643,000	234,707,000
Total assets													12,941,688,000	14,068,614,000
Liabilities and Stockholders’ Equity
Long-term debt - affiliated	[2]												500,000,000	500,000,000
Other liabilities													1,213,379,000	1,223,500,000
Total stockholders equity													8,450,553,000	9,569,535,000
Total liabilities and stockholders’ equity													12,941,688,000	14,068,614,000
CONSOLIDATED STATEMENTS OF INCOME
Interest and Dividend Income, Operating	[3]									575,833,000	131,067,000	602,830,000
Total investment related gains (losses), net										(5,392,000)	(5,187,000)	203,000
Operating Expenses										(35,912,000)	(20,517,000)	(20,742,000)
Interest and Debt Expense										(181,364,000)	(177,417,000)	(168,924,000)
Income (loss) before income tax and undistributed earnings of subsidiaries										353,165,000	(72,054,000)	413,367,000
Income Tax Expense (Benefit)										(37,294,000)	65,882,000	(23,911,000)
Net income (loss) before undistributed earnings of subsidiaries										390,459,000	(137,936,000)	437,278,000
Equity In Undistributed Earnings Of Subsidiaries										325,383,000	1,960,117,000	264,165,000
Net Income (Loss) Attributable to Parent										715,842,000	1,822,181,000	701,443,000
Other Comprehensive Income (Loss), Net of Tax										21,455,000	(7,672,000)	5,531,000
Total comprehensive income (loss)										737,297,000	1,814,509,000	706,974,000
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										715,842,000	1,822,181,000	701,443,000
Equity Net Earnings Subsidiaries										(325,383,000)	(1,960,117,000)	(264,165,000)
Other, net										36,754,000	57,677,000	(63,795,000)
Net cash provided by operating activities										427,213,000	(80,259,000)	373,483,000
Cash flows from investing activities
Sales of fixed maturity securities available-for-sale										482,324,000	514,508,000	228,383,000
Purchases of fixed maturity securities available-for-sale										(383,392,000)	(75,000,000)	(984,397,000)
Purchases Of Subsidiary Debt Securities										0	40,000,000	20,000,000
Payments for (Proceeds from) Short-term Investments										0	0	102,508,000
Payments for (Proceeds from) Other Investing Activities										44,000	125,506,000	(109,914,000)
Capital Contributions To Subsidiaries										(82,150,000)	(62,500,000)	(314,142,000)
Net cash used in investing activities										16,826,000	542,514,000	(1,057,562,000)
Cash flows from financing activities
Dividends to stockholders										(140,110,000)	(117,291,000)	(100,371,000)
Purchases of treasury stock										(299,679,000)	(43,508,000)	(122,916,000)
Exercise of stock options, net										3,459,000	7,292,000	15,321,000
Net Change In Cash Collateral For Loaned Securities										(2,244,000)	(37,290,000)	105,093,000
Maturities of Senior Debt										0	(300,000,000)	0
Proceeds from affiliated long-term debt issuance										0	0	(799,984,000)
Payments Of Debt Issuance Costs										0	0	(8,766,000)
Net cash used in financing activities										(438,574,000)	(490,797,000)	688,345,000
Change in cash and cash equivalents										5,465,000	(28,542,000)	4,266,000
Cash and cash equivalents, beginning of period					$ 15,176,000				$ 43,718,000	15,176,000	43,718,000	39,452,000
Cash and cash equivalents, end of period		$ 20,641,000				$ 15,176,000				20,641,000	15,176,000	43,718,000
Supplementary information:
Interest paid										176,236,000	185,554,000	169,860,000
Income taxes paid, net of refunds										93,000,000	8,248,000	1,500,000
Cash Acquired from Acquisition										0	$ 0	$ 529,000
Senior Notes Due 2017 [Member]
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]
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													$ 399,941,000	399,873,000
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													$ 399,941,000	399,873,000
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													399,466,000	399,245,000
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													$ 399,466,000	399,245,000
Senior Notes Due 2023 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													399,288,000	399,138,000
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													$ 399,289,000	399,138,000
Senior Notes Due 2026 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													399,988,000	399,987,000
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	399,987,000
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
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													318,743,000	318,740,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage													4.82%
Junior Subordinated Debentures Due 2065 [Member] | Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 398,674,000	$ 398,670,000

[1] Long-term debt - unaffiliated consists of the following: 2018 2017$400 million 6.45% Senior Notes due 2019 $399,941 $399,873$400 million 5.00% Senior Notes due 2021 399,466 399,245$400 million 4.70% Senior Notes due 2023 399,289 399,138$400 million 3.95% Senior Notes due 2026 399,987 399,987$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 398,674 398,670Subtotal 2,797,357 2,796,913Unamortized debt issue costs (19,601) (21,334)Total $2,777,756 $2,775,579Repayments of long-term debt—unaffiliated due over the next five years total $400,000 in 2019, $400,000 in 2021 and $400,000 in 2023.

[2] Long-term debt—affiliated in 2018 and 2017 and consists of $500,000 of subordinated debt issued to various operating subsidiaries.

[3] Interest/dividend income includes $450,000 and $478,602 of cash dividends received from consolidated subsidiaries in 2018 and 2016, respectively. In 2017 there were no cash dividends received from consolidated subsidiaries.