SCHEDULE II - Condensed Statements of Cash Flows - Parent Company Only (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cash flows from operating activities
Net (loss) income	$ (520,291,000)	$ 852,552,000	$ 1,536,854,000
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Other, net	8,647,000	47,959,000	33,414,000
Net cash provided by operating activities	573,635,000	1,226,920,000	1,098,601,000
Cash flows from investing activities
Sales and redemptions of short-term investments	336,456,000	270,087,000	201,479,000
Payments/ proceeds from trading securities [Line Items]
Purchases of short-term investments	331,432,000	141,157,000	182,211,000
Other, net	(186,823,000)	(185,965,000)	(118,473,000)
Net cash (used in) provided by investing activities	(1,080,378,000)	1,102,508,000	(446,676,000)
Cash flows from financing activities
Cash dividends paid to shareholders	(205,784,000)	(192,156,000)	(151,851,000)
Net Proceeds From Issuance Of Preferred Stock And Preference Stock	361,722,000	0	0
Repayment of debt	0	(200,000,000)	(200,000,000)
Repurchase of common shares	(413,737,000)	(1,065,121,000)	0
Issuance of common shares	16,041,000	37,682,000	16,034,000
Contract fees on forward sale agreement	0	(2,638,000)	(5,070,000)
Net cash used in financing activities	(241,758,000)	(922,233,000)	(765,231,000)
Effect of foreign exchange rate changes on cash	(20,326,000)	(34,420,000)	13,335,000
(Decrease) increase in cash and cash equivalents	(768,827,000)	1,372,775,000	(99,971,000)
Cash and cash equivalents - beginning of year	2,111,084,000	738,309,000	838,280,000
Cash and cash equivalents - end of year	1,342,257,000	2,111,084,000	738,309,000
Supplemental cash flow information:
Interest paid	49,259,000	42,995,000	32,476,000
[FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales	8,328,352,000	8,621,227,000	7,271,909,000
Purchases	10,549,343,000	8,572,471,000	9,039,313,000
[EquitySecuritiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales	730,929,000	607,459,000	688,360,000
Purchases	619,533,000	769,557,000	826,246,000
[ParentCompanyMember]
Cash flows from operating activities
Net (loss) income	(520,291,000)	852,552,000	1,536,854,000
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Equity in net (loss) income of subsidiaries	460,353,000	(970,206,000)	(1,636,801,000)
Other, net	7,663,000	36,388,000	8,203,000
Net cash provided by operating activities	(52,275,000)	(81,266,000)	(91,744,000)
Cash flows from investing activities
Sales and redemptions of short-term investments	154,473,000	69,489,000
Payments/ proceeds from trading securities [Line Items]
Purchases of short-term investments	99,955,000	0	0
Advances to/from subsidiaries, net	3,511,000	697,019,000	508,213,000
Net issue of intercompany loans receivable and payable	379,676,000	380,771,000
Investments in subsidiaries	(860,000,000)		(175,111,000)
Other, net	2,408,000	(4,283,000)	(2,851,000)
Foreign exchange forward contracts	(6,750,000)	3,528,000	(71,303,000)
Net cash (used in) provided by investing activities	19,815,000	1,844,917,000	258,948,000
Cash flows from financing activities
Cash dividends paid to shareholders	(205,784,000)	(192,156,000)	(151,851,000)
Net Proceeds From Issuance Of Preferred Stock And Preference Stock		0	0
Repayment of debt	0	(200,000,000)	(200,000,000)
Repurchase of common shares	(413,737,000)	(1,065,121,000)
Issuance of common shares	16,041,000	37,682,000	16,034,000
Contract fees on forward sale agreement	0	(2,638,000)	(5,070,000)
Net cash used in financing activities	(241,758,000)	(1,422,233,000)	(340,887,000)
Effect of foreign exchange rate changes on cash	(3,107,000)	(15,867,000)	(1,342,000)
(Decrease) increase in cash and cash equivalents	(277,325,000)	325,551,000	(175,025,000)
Cash and cash equivalents - beginning of year	329,054,000	3,503,000	178,528,000
Cash and cash equivalents - end of year	51,729,000	329,054,000	3,503,000
Supplemental cash flow information:
Interest paid	743,000	1,680,000	14,362,000
Non-cash dividends received from subsidiary	274,000,000	500,000,000	600,000,000
[ParentCompanyMember] | [FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales	$ 446,452,000	$ 698,393,000