SCHEDULE II - Condensed Statements of Cash Flows - Parent Company Only (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Cash flows from operating activities
Net income (loss)	$ 1,134,514,000		$ (520,291,000)		$ 852,552,000
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Other, net	52,246,000		8,647,000		47,959,000
Net cash provided by operating activities	693,293,000		573,635,000		1,226,920,000
Cash flows from investing activities
Sales and redemptions of short-term investments	110,360,000		336,456,000		270,087,000
Payments/ proceeds from trading securities [Line Items]
Purchases of short-term investments	215,473,000		331,432,000		141,157,000
Other, net	995,000		(186,823,000)		(185,965,000)
Net cash (used in) provided by investing activities	(219,538,000)		(1,080,378,000)		1,102,508,000
Cash flows from financing activities
Cash dividends paid to shareholders	(217,615,000)		(205,784,000)		(192,156,000)
Net Proceeds From Issuance Of Preferred Stock And Preference Stock	0		361,722,000		0
Repayment of debt	0		0		(200,000,000)
Repurchase of common shares	(504,991,000)		(413,737,000)		(1,065,121,000)
Issuance of common shares	34,323,000		16,041,000		37,682,000
Contract fees on forward sale agreement	0		0		(2,638,000)
Net cash used in financing activities	(688,283,000)		(241,758,000)		(922,233,000)
Effect of foreign exchange rate changes on cash	(6,024,000)		(20,326,000)		(34,420,000)
(Decrease) increase in cash and cash equivalents	(220,552,000)		(768,827,000)		1,372,775,000
Cash and cash equivalents - beginning of year	1,342,257,000		2,111,084,000		738,309,000
Cash and cash equivalents - end of year	1,121,705,000		1,342,257,000		2,111,084,000
Supplemental cash flow information:
Interest paid	49,259,000		49,259,000		42,995,000
[FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	6,969,074,000		8,328,352,000		8,621,227,000
Purchases of fixed maturities and equities	8,067,087,000		10,549,343,000		8,572,471,000
[EquitySecuritiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	821,977,000		730,929,000		607,459,000
Purchases of fixed maturities and equities	830,323,000		619,533,000		769,557,000
[ParentCompanyMember]
Cash flows from operating activities
Net income (loss)	1,134,514,000		(520,291,000)		852,552,000
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Equity in net income (loss) of subsidiaries	(1,215,106,000)		460,353,000		(970,206,000)
Other, net	30,573,000		7,663,000		36,388,000
Net cash provided by operating activities	(50,019,000)		(52,275,000)		(81,266,000)
Cash flows from investing activities
Sales and redemptions of short-term investments	8,543,000		154,473,000		69,489,000
Payments/ proceeds from trading securities [Line Items]
Purchases of short-term investments	0		99,955,000		0
Advances to/from subsidiaries, net	190,017,000		3,511,000		697,019,000
Net issue of intercompany loans receivable and payable	132,797,000		379,676,000		380,771,000
Investments in subsidiaries	0		(860,000,000)		0
Dividends received from subsidiaries	(200,000,000)		0		0
Other, net	772,000		2,408,000		(4,283,000)
Foreign exchange forward contracts	3,000		(6,750,000)		3,528,000
Net cash (used in) provided by investing activities	716,648,000		19,815,000		1,844,917,000
Cash flows from financing activities
Cash dividends paid to shareholders	(217,615,000)		(205,784,000)		(192,156,000)
Net Proceeds From Issuance Of Preferred Stock And Preference Stock	0		361,722,000		0
Repayment of debt	0		0		(200,000,000)
Repurchase of common shares	(504,991,000)		(413,737,000)		(1,065,121,000)
Issuance of common shares	34,323,000		16,041,000		37,682,000
Contract fees on forward sale agreement	0		0		(2,638,000)
Net cash used in financing activities	(688,283,000)		(241,758,000)		(1,422,233,000)
Effect of foreign exchange rate changes on cash	297,000		(3,107,000)		(15,867,000)
(Decrease) increase in cash and cash equivalents	(21,357,000)		(277,325,000)		325,551,000
Cash and cash equivalents - beginning of year	51,729,000	[1]	329,054,000	[1],[2]	3,503,000
Cash and cash equivalents - end of year	30,372,000	[1]	51,729,000	[1]	329,054,000	[1],[2]
Supplemental cash flow information:
Interest paid	579,000		743,000		1,680,000
Non-cash dividends received from subsidiary	200,000,000		274,000,000		500,000,000
[ParentCompanyMember] | [FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	$ 184,516,000		$ 446,452,000		$ 698,393,000

[1] The Company received non-cash dividends from its subsidiaries of $200 million, $274 million and $500 million for the years ended December 31, 2012, 2011 and 2010, respectively, which have been excluded from the Condensed Statements of Cash Flows—Parent Company Only.

[2] During 2010, the Company sold a wholly-owned subsidiary to another wholly-owned subsidiary and received fixed maturities and short-term investments as partial settlement of certain intercompany loans receivable. These non-cash transactions have been excluded from the Condensed Statements of Cash Flows—Parent Company Only.