Condensed Financial Information of The Registrant (Details) (USD $)	3 Months Ended										12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011	Jun. 30, 2006	Aug. 21, 2012	Sep. 19, 2013
Assets
Fixed maturity securities available-for-sale	$ 25,480,972,000					$ 21,474,136,000					$ 25,480,972,000		$ 21,474,136,000
Short-term investments	97,694,000					139,395,000					97,694,000		139,395,000
Cash and cash equivalents	1,645,669,000					923,647,000					1,645,669,000		923,647,000		1,259,892,000
Other assets	628,268,000					489,972,000					628,268,000		489,972,000
Total assets	44,679,611,000					39,674,473,000					44,679,611,000		39,674,473,000
Liabilities and Stockholders’ Equity
Other liabilities	994,230,000					541,035,000					994,230,000		541,035,000
Total stockholders equity	7,023,452,000					5,935,527,000					7,023,452,000		5,935,527,000		6,910,187,000		5,818,698,000
Total liabilities and stockholders’ equity	44,679,611,000					39,674,473,000					44,679,611,000		39,674,473,000
CONSOLIDATED STATEMENTS OF INCOME
Total investment related gains (losses), net											186,193,000		63,990,000		254,136,000
Income Tax Expense (Benefit)											324,486,000		216,417,000		287,330,000
Net income	191,091,000		157,996,000	198,296,000	136,664,000	144,959,000		137,955,000	(49,612,000)	185,535,000	684,047,000		418,837,000		631,893,000
Other Comprehensive Income (Loss), Net of Tax											651,522,000		(1,103,295,000)		490,749,000
Total comprehensive income (loss)											1,335,569,000		(684,458,000)		1,122,642,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount																		850,000,000.0
Cash flows from operating activities
Other, net											155,428,000		99,634,000		85,523,000
Net cash provided by operating activities											2,336,155,000		1,727,160,000		1,974,527,000
Cash flows from investing activities
Sales of fixed maturity securities available-for-sale											4,309,985,000		3,629,378,000		5,465,014,000
Purchases of fixed maturity securities available-for-sale											63,785,000		33,724,000		40,466,000
Payments to Acquire Businesses, Net of Cash Acquired											0		(2,805,000)		0
Payments for (Proceeds from) Short-term Investments											38,060,000		138,024,000		(101,214,000)
Payments for (Proceeds from) Other Investing Activities											286,665,000		(209,900,000)		(216,274,000)
Net cash used in investing activities											(1,310,945,000)		(1,335,101,000)		(1,967,996,000)
Cash flows from financing activities
Dividends to stockholders											(87,256,000)		(77,642,000)		(61,945,000)
Purchases of treasury stock											(201,525,000)		(269,204,000)		(6,924,000)
Excess tax benefits from share-based payment arrangement cash flow											(3,011,000)		3,125,000		416,000
Exercise of stock options, net											9,246,000		28,390,000		(3,087,000)
Proceeds from affiliated long-term debt issuance											(100,000,000)		(398,492,000)		(400,000,000)
Payments Of Debt Issuance Costs											(4,260,000)		(3,400,000)		(6,255,000)
Net cash (used in) provided by financing activities											(255,559,000)		(681,213,000)		281,939,000
Change in cash and cash equivalents											722,022,000		(336,245,000)		297,022,000
Cash and cash equivalents, beginning of period					923,647,000					1,259,892,000	923,647,000		1,259,892,000		962,870,000
Cash and cash equivalents, end of period	1,645,669,000					923,647,000					1,645,669,000		923,647,000		1,259,892,000
Supplementary information:
Cash paid for interest											136,499,000		116,809,000		100,984,000
Cash paid for income taxes, net of refunds											70,342,000		110,773,000		97,000,000
Parent Company [Member]
Assets
Fixed maturity securities available-for-sale	486,813,000					502,455,000					486,813,000		502,455,000
Short-term investments	87,726,000					261,853,000					87,726,000		261,853,000
Cash and cash equivalents	48,819,000					24,137,000					48,819,000		24,137,000		52,680,000
Investments In Subsidiary	8,649,084,000					7,534,714,000					8,649,084,000		7,534,714,000
Loans and Leases Receivable, Related Parties	1,060,000,000					1,101,751,000					1,060,000,000		1,101,751,000
Other assets	360,059,000					204,887,000					360,059,000		204,887,000
Total assets	10,692,501,000					9,629,797,000					10,692,501,000		9,629,797,000
Liabilities and Stockholders’ Equity
Short-term and long-term debt - unaffiliated	2,294,993,000	[1]				2,294,278,000	[1]				2,294,993,000	[1]	2,294,278,000	[1]
Long-term debt - affiliated	500,000,000	[2]				500,000,000	[2]				500,000,000	[2]	500,000,000	[2]
Other liabilities	874,056,000					899,992,000					874,056,000		899,992,000
Total stockholders equity	7,023,452,000					5,935,527,000					7,023,452,000		5,935,527,000
Total liabilities and stockholders’ equity	10,692,501,000					9,629,797,000					10,692,501,000		9,629,797,000
CONSOLIDATED STATEMENTS OF INCOME
Interest and Dividend Income, Operating											521,623,000	[3]	275,215,000	[3]	86,396,000	[3]
Total investment related gains (losses), net											4,936,000		1,714,000		4,515,000
Operating Expenses											(10,751,000)		(21,164,000)		(26,431,000)
Interest and Debt Expense											(131,852,000)		(162,212,000)		(143,260,000)
Income (loss) before income tax and undistributed earnings of subsidiaries											383,956,000		93,553,000		(78,780,000)
Income Tax Expense (Benefit)											(22,008,000)		33,850,000		(9,566,000)
Net income (loss) before undistributed earnings of subsidiaries											405,964,000		59,703,000		(69,214,000)
Equity In Undistributed Earnings Of Subsidiaries											278,083,000		359,134,000		701,107,000
Net income											684,047,000		418,837,000		631,893,000
Other Comprehensive Income (Loss), Net of Tax											36,876,000		21,033,000		9,984,000
Total comprehensive income (loss)											720,923,000		439,870,000		641,877,000
Debt Disclosure [Abstract]
Proceeds from Dividends Received											315,000,000		175,000,000
Long-term Debt	500,000,000					500,000,000					500,000,000		500,000,000
Cash flows from operating activities
Equity Net Earnings Subsidiaries											(278,083,000)		(359,134,000)		(701,107,000)
Other, net											(171,299,000)		162,586,000		134,232,000
Net cash provided by operating activities											234,665,000		222,289,000		65,018,000
Cash flows from investing activities
Sales of fixed maturity securities available-for-sale											132,732,000		176,062,000		122,212,000
Purchases of fixed maturity securities available-for-sale											(105,535,000)		(103,566,000)		(213,548,000)
Purchases Of Subsidiary Debt Securities											41,751,000		(76,751,000)		(250,000,000)
Payments to Acquire Businesses, Net of Cash Acquired											0		(2,805,000)		0
Payments for (Proceeds from) Short-term Investments											96,967,000		(96,967,000)		0
Payments for (Proceeds from) Other Investing Activities											126,397,000		(79,023,000)		5,718,000
Capital Contributions To Subsidiaries											(222,760,000)		(144,459,000)		(70,431,000)
Net cash used in investing activities											69,552,000		(327,509,000)		(406,049,000)
Cash flows from financing activities
Dividends to stockholders											(87,256,000)		(77,642,000)		(61,945,000)
Purchases of treasury stock											(201,525,000)		(269,204,000)		(6,924,000)
Excess tax benefits from share-based payment arrangement cash flow											0		0		416,000
Exercise of stock options, net											9,246,000		28,390,000		(3,087,000)
Proceeds from affiliated long-term debt issuance											0		(398,533,000)		(400,000,000)
Payments Of Debt Issuance Costs											0		(3,400,000)		(6,255,000)
Net cash (used in) provided by financing activities											(279,535,000)		76,677,000		322,205,000
Change in cash and cash equivalents											24,682,000		(28,543,000)		(18,826,000)
Cash and cash equivalents, beginning of period					24,137,000					52,680,000	24,137,000		52,680,000		71,506,000
Cash and cash equivalents, end of period	48,819,000					24,137,000					48,819,000		24,137,000		52,680,000
Supplementary information:
Cash paid for interest											161,499,000		141,615,000		130,047,000
Cash paid for income taxes, net of refunds											87,000		82,000,000		30,500,000
Junior Subordinated Debentures Due 2065 [Member]
Debt Disclosure [Abstract]
Debt Instrument, Face Amount	400,000,000										400,000,000
Debt Instrument, Interest Rate, Stated Percentage	6.75%										6.75%
Junior Subordinated Debentures Due 2065 [Member] | Parent Company [Member]
Liabilities and Stockholders’ Equity
Short-term and long-term debt - unaffiliated	398,660,000					398,657,000					398,660,000		398,657,000
Subordinated Debentures Due 2042 [Member]
Debt Disclosure [Abstract]
Debt Instrument, Face Amount	400,000,000										400,000,000								400,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	6.20%										6.20%								6.20%
Subordinated Debentures Due 2042 [Member] | Parent Company [Member]
Liabilities and Stockholders’ Equity
Short-term and long-term debt - unaffiliated	400,000,000					400,000,000					400,000,000		400,000,000
Senior Notes Due 2023 [Member]
Debt Disclosure [Abstract]
Debt Instrument, Face Amount	400,000,000										400,000,000									400,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	4.70%										4.70%									4.70%
Senior Notes Due 2023 [Member] | Parent Company [Member]
Liabilities and Stockholders’ Equity
Short-term and long-term debt - unaffiliated	398,684,000					398,533,000					398,684,000		398,533,000
Senior Notes Due 2021 [Member]
Debt Disclosure [Abstract]
Debt Instrument, Face Amount	400,000,000										400,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.00%										5.00%
Senior Notes Due 2021 [Member] | Parent Company [Member]
Liabilities and Stockholders’ Equity
Short-term and long-term debt - unaffiliated	398,583,000					398,362,000					398,583,000		398,362,000
Senior Notes Due 2019 [Member]
Debt Disclosure [Abstract]
Debt Instrument, Face Amount	400,000,000										400,000,000
Debt Instrument, Interest Rate, Stated Percentage	6.45%										6.45%
Senior Notes Due 2019 [Member] | Parent Company [Member]
Liabilities and Stockholders’ Equity
Short-term and long-term debt - unaffiliated	399,669,000					399,602,000					399,669,000		399,602,000
Debt Disclosure [Abstract]
Repayments of Long-term Debt											400,000,000
Senior Notes Due 2017 [Member]
Debt Disclosure [Abstract]
Debt Instrument, Face Amount	300,000,000										300,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.625%										5.625%
Senior Notes Due 2017 [Member] | Parent Company [Member]
Liabilities and Stockholders’ Equity
Short-term and long-term debt - unaffiliated	299,397,000					299,124,000					299,397,000		299,124,000
Debt Disclosure [Abstract]
Repayments of Long-term Debt											$ 300,000,000

[1] Long-term debt - unaffiliated consists of the following: 2014 2013$400 million 6.75% Junior Subordinated Debentures due 2065 $398,660 $398,657$400 million 6.20% Subordinated Debentures due 2042 400,000 400,000$400 million 4.70% Senior Notes due 2023 398,684 398,533$400 million 5.00% Senior Notes due 2021 398,583 398,362$400 million 6.45% Senior Notes due 2019 399,669 399,602$300 million 5.625% Senior Notes due 2017 299,397 299,124Total $2,294,993 $2,294,278Repayments of long-term debt—unaffiliated due over the next five years total $300,000, in 2017 and $400,000 in 2019.

[2] Long-term debt—affiliated in 2014 and 2013 and consists of $500,000 of subordinated debt issued to various operating subsidiaries.

[3] Interest/Dividend income includes $423,323 and $175,000 of cash dividends received from consolidated subsidiaries in 2014 and 2013, respectively. Cash dividends received from consolidated subsidiaries in 2012 were not material.