Schedule II Condensed Financial Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets
Fixed maturity securities available-for-sale	$ 22,291,614,000				$ 16,200,950,000				$ 22,291,614,000	$ 16,200,950,000
Short-term investments	288,082,000				88,566,000				288,082,000	88,566,000
Cash and cash equivalents	1,259,892,000				962,870,000				1,259,892,000	962,870,000	463,661,000
Other assets	390,757,000				332,466,000				390,757,000	332,466,000
Total assets	40,360,438,000				31,633,973,000				40,360,438,000	31,633,973,000
Liabilities and Stockholders Equity
Other liabilities	742,249,000				810,775,000				742,249,000	810,775,000
Total stockholders equity	6,910,187,000				5,818,698,000				6,910,187,000	5,818,698,000	4,765,406,000	3,639,754,000
Total liabilities and stockholders equity	40,360,438,000				31,633,973,000				40,360,438,000	31,633,973,000
CONSOLIDATED STATEMENTS OF INCOME
Total investment related gains (losses), net									254,136,000	(36,056,000)	212,030,000
Provision for income taxes									287,330,000	217,526,000	270,439,000
Net income	222,989,000	144,475,000	141,111,000	123,318,000	138,579,000	134,602,000	123,944,000	148,920,000	631,893,000	546,045,000	535,742,000
Cash Flows from Operating Activities:
Other, net									85,523,000	134,835,000	30,359,000
Net cash provided by operating activities									1,974,527,000	1,309,532,000	1,842,740,000
Cash Flows from Investing Activities:
Sales of fixed maturity securities available-for-sale									5,465,014,000	3,165,479,000	3,319,453,000
Purchases of fixed maturity securities available-for-sale									6,818,378,000	4,011,985,000	4,854,416,000
Change in short-term investments and other invested assets									317,488,000	91,880,000	37,395,000
Net cash used in investing activities									(1,967,996,000)	(905,781,000)	(1,720,482,000)
Cash Flows from Financing Activities:
Dividends to stockholders									61,945,000	44,229,000	35,170,000
Purchases of treasury stock									6,924,000	380,345,000	718,000
Excess tax benefits from share-based payment arrangement cash flow									416,000	4,933,000	(2,255,000)
Exercise of stock options, net									(3,087,000)	6,449,000	2,277,000
Change in cash collateral for derivative positions									(132,933,000)	231,180,000	26,324,000
Principal payments on debt									0	0	0
Maturity of trust preferred securities									0	159,473,000	0
Proceeds from affiliated long-term debt issuance									400,000,000	397,788,000	0
PaymentsOfDebtIssuanceCosts									6,255,000	3,400,000	0
Proceeds from redemption and remarketing of trust preferred securities									0	154,588,000	0
Net cash used in financing activities									281,939,000	101,995,000	(193,429,000)
Change in cash and cash equivalents									297,022,000	499,209,000	(48,366,000)
Cash and cash equivalents, beginning of period				962,870,000				463,661,000	962,870,000	463,661,000	512,027,000
Cash and cash equivalents, end of period	1,259,892,000				962,870,000				1,259,892,000	962,870,000	463,661,000
Supplementary information:
Cash paid for interest									100,984,000	100,733,000	95,939,000
Cash paid for income taxes, net of refunds									97,000,000	129,009,000	10,452,000
Parent Company [Member]
Assets
Fixed maturity securities available-for-sale	604,896,000				467,306,000				604,896,000	467,306,000
Short-term investments	64,681,000				44,825,000				64,681,000	44,825,000
Cash and cash equivalents	52,680,000				71,506,000				52,680,000	71,506,000	31,698,000
Investments In Subsidiary	8,148,532,000				6,879,608,000				8,148,532,000	6,879,608,000
Loans and Leases Receivable, Related Parties	1,025,000,000				775,000,000				1,025,000,000	775,000,000
Other assets	155,190,000				132,841,000				155,190,000	132,841,000
Total assets	10,050,979,000				8,371,086,000				10,050,979,000	8,371,086,000
Liabilities and Stockholders Equity
Short-term and long-term debt - unaffiliated	1,895,181,000				1,494,615,000				1,895,181,000	1,494,615,000
Long-term debt - affiliated	500,000,000				500,000,000				500,000,000	500,000,000
Other liabilities	745,611,000				557,773,000				745,611,000	557,773,000
Total stockholders equity	6,910,187,000				5,818,698,000				6,910,187,000	5,818,698,000
Total liabilities and stockholders equity	10,050,979,000				8,371,086,000				10,050,979,000	8,371,086,000
CONSOLIDATED STATEMENTS OF INCOME
Interest and Dividend Income, Operating									86,396,000	245,631,000	128,448,000
Total investment related gains (losses), net									4,515,000	1,723,000	2,944,000
Operating Expenses									(26,431,000)	(32,550,000)	(19,442,000)
Interest and Debt Expense									(143,260,000)	(111,600,000)	(96,556,000)
Income (loss) before income tax and undistributed earnings of subsidiaries									(78,780,000)	103,204,000	15,394,000
Provision for income taxes									(9,566,000)	8,935,000	6,707,000
Net income (loss) before undistributed earnings of subsidiaries									(69,214,000)	94,269,000	8,687,000
Equity In Undistributed Eanings Of Subsidiaries									701,107,000	451,776,000	527,055,000
Net income									631,893,000	546,045,000	535,742,000
Cash Flows from Operating Activities:
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									631,893,000	546,045,000	535,742,000
Equity Net Earnings Subsidiaries									(701,107,000)	(453,806,000)	(528,228,000)
Loss On Retirement Of Trust Preferred Securities									0	4,391,000	0
Other, net									134,232,000	143,716,000	12,205,000
Net cash provided by operating activities									65,018,000	240,346,000	19,719,000
Cash Flows from Investing Activities:
Sales of fixed maturity securities available-for-sale									122,212,000	171,791,000	169,910,000
Purchases of fixed maturity securities available-for-sale									(213,548,000)	(116,010,000)	(214,025,000)
Purchases Of Subsidiary Debt Securities									(250,000,000)	(475,000,000)	(100,000,000)
Maturity Of Subsidiary Debt Securities									0	50,000,000	226,550,000
Change in short-term investments and other invested assets									5,718,000	(2,055,000)	7,688,000
Captial Contributions To Subsidiaries									(70,431,000)	(105,575,000)	(73,950,000)
Net cash used in investing activities									(406,049,000)	(476,849,000)	16,173,000
Cash Flows from Financing Activities:
Dividends to stockholders									(61,945,000)	(44,229,000)	(35,170,000)
Purchases of treasury stock									(6,924,000)	(380,345,000)	(718,000)
Excess tax benefits from share-based payment arrangement cash flow									416,000	4,933,000	(2,255,000)
Exercise of stock options, net									(3,087,000)	6,449,000	2,277,000
Change in cash collateral for derivative positions									0	0	6,759,000
Principal payments on debt									0	(200,000,000)	0
Maturity of trust preferred securities									0	(159,473,000)	0
Proceeds from affiliated long-term debt issuance									0	500,000,000	0
Proceeds from unaffiliated long-term debt issuance									400,000,000	397,788,000	0
PaymentsOfDebtIssuanceCosts									(6,255,000)	(3,400,000)	0
Proceeds from redemption and remarketing of trust preferred securities									0	154,588,000	0
Net cash used in financing activities									322,205,000	276,311,000	(29,107,000)
Change in cash and cash equivalents									(18,826,000)	39,808,000	6,785,000
Cash and cash equivalents, beginning of period				71,506,000				31,698,000	71,506,000	31,698,000	24,913,000
Cash and cash equivalents, end of period	52,680,000				71,506,000				52,680,000	71,506,000	31,698,000
Supplementary information:
Cash paid for interest									130,047,000	98,809,000	96,760,000
Cash paid for income taxes, net of refunds									$ 30,500,000	$ 0	$ (37,200,000)