Condensed Financial Information of The Registrant (Details) - USD ($)		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	Dec. 31, 2013	Jun. 30, 2006
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													$ 3,110,853,000	$ 2,312,629,000
Debt Issuance Cost													(22,218,000)	(15,081,000)
Long-term debt													3,088,635,000	2,297,548,000
Assets
Fixed maturity securities available-for-sale													32,093,625,000	29,642,905,000
Short-term investments													76,710,000	558,284,000
Cash and cash equivalents		$ 1,200,718,000			$ 1,525,275,000	$ 1,525,275,000			$ 1,645,669,000	$ 1,525,275,000	$ 1,645,669,000	$ 923,647,000	1,200,718,000	1,525,275,000	$ 1,645,669,000	$ 923,647,000
Other assets													755,338,000	712,366,000
Total assets													53,097,879,000	50,383,152,000
Liabilities and Stockholders’ Equity
Other liabilities													1,041,880,000	1,165,071,000
Total stockholders equity													7,093,082,000	6,135,381,000	7,023,452,000	5,935,527,000
Total liabilities and stockholders’ equity													53,097,879,000	50,383,152,000
CONSOLIDATED STATEMENTS OF INCOME
Total investment related gains (losses), net										94,195,000	(164,750,000)	186,193,000
Income Tax Expense (Benefit)										342,503,000	242,629,000	324,486,000
Net Income (Loss) Attributable to Parent		190,149,000	$ 198,719,000	$ 236,103,000	76,472,000	163,127,000	$ 83,534,000	$ 130,391,000	125,114,000	701,443,000	502,166,000	684,047,000
Other Comprehensive Income (Loss), Net of Tax										431,045,000	(948,845,000)	651,522,000
Total comprehensive income (loss)										1,132,488,000	(446,679,000)	1,335,569,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount																	$ 850,000,000.0
Cash flows from operating activities
Net Income (Loss) Attributable to Parent		190,149,000	$ 198,719,000	$ 236,103,000	76,472,000	163,127,000	$ 83,534,000	$ 130,391,000	125,114,000	701,443,000	502,166,000	684,047,000
Other, net										117,949,000	(49,564,000)	140,119,000
Net cash provided by operating activities										1,465,718,000	2,088,615,000	2,336,155,000
Cash flows from investing activities
Sales of fixed maturity securities available-for-sale										4,584,828,000	5,461,687,000	4,309,985,000
Purchases of fixed maturity securities available-for-sale										88,840,000	21,322,000	63,785,000
Payments to Acquire Businesses, Net of Cash Acquired										0	(145,235,000)	0
Payments for (Proceeds from) Short-term Investments										465,628,000	(470,002,000)	38,060,000
Payments for (Proceeds from) Other Investing Activities										(97,790,000)	91,960,000	(2,573,000)
Net cash used in investing activities										(2,825,726,000)	(1,431,741,000)	(1,310,945,000)
Cash flows from financing activities
Dividends to stockholders										(100,371,000)	(93,381,000)	(87,256,000)
Purchases of treasury stock										(122,916,000)	(384,519,000)	(201,525,000)
Excess tax benefits from share-based payment arrangement cash flow										162,000	2,963,000	(3,011,000)
Exercise of stock options, net										15,321,000	11,151,000	9,246,000
Proceeds from affiliated long-term debt issuance										(799,984,000)	0	(100,000,000)
Payments Of Debt Issuance Costs										(8,766,000)	(4,748,000)	(4,260,000)
Net cash used in financing activities										1,055,389,000	(708,282,000)	(255,559,000)
Change in cash and cash equivalents										(324,557,000)	(120,394,000)	722,022,000
Cash and cash equivalents, beginning of period					1,525,275,000				1,645,669,000	1,525,275,000	1,645,669,000	923,647,000
Cash and cash equivalents, end of period		1,200,718,000				1,525,275,000				1,200,718,000	1,525,275,000	1,645,669,000
Supplementary information:
Interest paid										156,727,000	148,124,000	136,499,000
Income taxes paid, net of refunds										61,085,000	41,577,000	70,342,000
Parent Company [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													3,096,413,000	2,295,709,000
Debt Issuance Cost													(23,164,000)	(16,046,000)
Long-term debt	[1]												3,073,249,000	2,279,663,000
Assets
Fixed maturity securities available-for-sale													1,154,559,000	426,218,000
Short-term investments													245,478,000	254,398,000
Cash and cash equivalents		43,718,000			39,452,000	39,452,000			48,819,000	39,452,000	48,819,000	24,137,000	43,718,000	39,452,000	$ 48,819,000	$ 24,137,000
Investments In Subsidiary													9,209,700,000	8,110,687,000
Loans and Leases Receivable, Related Parties													1,050,000,000	1,070,000,000
Other assets													258,379,000	309,340,000
Total assets													11,961,834,000	10,210,095,000
Liabilities and Stockholders’ Equity
Long-term debt - affiliated	[2]												500,000,000	500,000,000
Other liabilities													1,295,503,000	1,295,051,000
Total stockholders equity													7,093,082,000	6,135,381,000
Total liabilities and stockholders’ equity													11,961,834,000	10,210,095,000
CONSOLIDATED STATEMENTS OF INCOME
Interest and Dividend Income, Operating	[3]									602,830,000	321,645,000	521,623,000
Total investment related gains (losses), net										203,000	(324,000)	4,936,000
Operating Expenses										(20,742,000)	(13,652,000)	(10,751,000)
Interest and Debt Expense										(168,924,000)	(176,364,000)	(131,852,000)
Income (loss) before income tax and undistributed earnings of subsidiaries										413,367,000	131,305,000	383,956,000
Income Tax Expense (Benefit)										(23,911,000)	(19,465,000)	(22,008,000)
Net income (loss) before undistributed earnings of subsidiaries										437,278,000	150,770,000	405,964,000
Equity In Undistributed Earnings Of Subsidiaries										264,165,000	351,396,000	278,083,000
Net Income (Loss) Attributable to Parent										701,443,000	502,166,000	684,047,000
Other Comprehensive Income (Loss), Net of Tax										5,531,000	44,073,000	36,876,000
Total comprehensive income (loss)										706,974,000	546,239,000	720,923,000
Debt Disclosure [Abstract]
Proceeds from Dividends Received										196,445,000	423,323,000	175,000,000
Long-term Debt													500,000,000	500,000,000
Cash flows from operating activities
Net Income (Loss) Attributable to Parent										701,443,000	502,166,000	684,047,000
Equity Net Earnings Subsidiaries										(264,165,000)	(351,396,000)	(278,083,000)
Other, net										(63,795,000)	486,159,000	(171,299,000)
Net cash provided by operating activities										373,483,000	636,929,000	234,665,000
Cash flows from investing activities
Sales of fixed maturity securities available-for-sale										228,383,000	100,734,000	132,732,000
Purchases of fixed maturity securities available-for-sale										(984,397,000)	(52,698,000)	(105,535,000)
Purchases Of Subsidiary Debt Securities										20,000,000	(10,000,000)	41,751,000
Payments to Acquire Businesses, Net of Cash Acquired										0	(3,701,000)	0
Payments for (Proceeds from) Short-term Investments										102,508,000	(102,508,000)	96,967,000
Payments for (Proceeds from) Other Investing Activities										(109,914,000)	(7,542,000)	126,397,000
Capital Contributions To Subsidiaries										(314,142,000)	(103,832,000)	(222,760,000)
Net cash used in investing activities										(1,057,562,000)	(179,547,000)	69,552,000
Cash flows from financing activities
Dividends to stockholders										(100,371,000)	(93,381,000)	(87,256,000)
Purchases of treasury stock										(122,916,000)	(384,519,000)	(201,525,000)
Exercise of stock options, net										15,321,000	11,151,000	9,246,000
Net Change In Cash Collateral For Loaned Securities										105,093,000	0	0
Proceeds from affiliated long-term debt issuance										(799,984,000)	0	0
Payments Of Debt Issuance Costs										(8,766,000)	0	0
Net cash used in financing activities										688,345,000	(466,749,000)	(279,535,000)
Change in cash and cash equivalents										4,266,000	(9,367,000)	24,682,000
Cash and cash equivalents, beginning of period					$ 39,452,000				$ 48,819,000	39,452,000	48,819,000	24,137,000
Cash and cash equivalents, end of period		$ 43,718,000				$ 39,452,000				43,718,000	39,452,000	48,819,000
Supplementary information:
Interest paid										169,860,000	165,775,000	161,499,000
Income taxes paid, net of refunds										1,500,000	$ (120,680,000)	$ 87,000
Senior Notes Due 2017 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													299,945,000	299,671,000
Debt Disclosure [Abstract]
Debt Instrument, Face Amount													$ 300,000,000.0
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													$ 299,945,000	299,671,000
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													399,805,000	399,737,000
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													$ 399,805,000	399,737,000
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													399,025,000	398,803,000
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													$ 399,025,000	398,803,000
Senior Notes Due 2023 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													398,986,000	398,835,000
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													$ 398,986,000	398,835,000
Senior Notes Due 2026 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													399,985,000	0
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													$ 399,985,000	0
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	0
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	0
Junior Subordinated Debentures Due 2065 [Member]
Supplemental Condensed Financial Information Of The Registrant [Line Items]
Long-term Debt, Gross													318,737,000	318,734,000
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													$ 398,667,000	$ 398,663,000

[1] Long-term debt - unaffiliated consists of the following: 2016 2015$300 million 5.625% Senior Notes due 2017 $299,945 $299,671$400 million 6.45% Senior Notes due 2019 399,805 399,737$400 million 5.00% Senior Notes due 2021 399,025 398,803$400 million 4.70% Senior Notes due 2023 398,986 398,835$400 million 3.95% Senior Notes due 2026 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 million Variable Rate Junior Subordinated Debentures due 2065 398,667 398,663Subtotal 3,096,413 2,295,709Unamortized debt issue costs (23,164) (16,046)Total $3,073,249 $2,279,663Repayments of long-term debt—unaffiliated due over the next five years total $300,000 in 2017, $400,000 in 2019 and $400,000 in 2021.

[2] Long-term debt—affiliated in 2016 and 2015 and consists of $500,000 of subordinated debt issued to various operating subsidiaries.

[3] nterest/Dividend income includes $478,602, $196,445 and $423,323 of cash dividends received from consolidated subsidiaries in 2016, 2015 and 2014, respectively.