Unaudited Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Cash flows from operating activities
Net income (loss)	$ 536,287	$ (682,771)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Amortization of net premium on investments	64,054	37,494
Amortization of intangible assets	17,786	17,992
Net realized and unrealized investment (gains) losses	(230,867)	34,000
Changes in:
Reinsurance balances, net	(281,652)	(361,501)
Reinsurance recoverable on paid and unpaid losses, net of ceded premiums payable	33,925	11,481
Funds held by reinsured companies and funds held - directly managed	36,609	552,361
Deferred acquisition costs	(81,990)	(31,484)
Net tax assets and liabilities	50,431	(32,493)
Unpaid losses and loss expenses including life policy benefits	(494,083)	853,057
Unearned premiums	528,623	354,853
Other net changes in operating assets and liabilities	3,445	(10,651)
Net cash provided by operating activities	182,568	742,338
Cash flows from investing activities
Redemptions Of Fixed Maturities	512,544	801,285
Payments/ proceeds from trading securities [Line Items]
Sales and redemptions of short-term investments	52,804	76,061
Purchases of short-term investments	(42,046)	(240,207)
Other, net	16,116	(17,820)
Net cash provided by (used in) investing activities	324,076	(1,307,078)
Cash flows from financing activities
Cash dividends paid to shareholders	(110,735)	(95,009)
Net proceeds from issuance of preferred shares	0	361,722
Repurchase of common shares	(221,995)	(244,222)
Issuance of common shares	16,036	13,125
Net cash (used in) provided by financing activities	(316,694)	35,616
Effect of foreign exchange rate changes on cash	(19,789)	24,882
Increase (decrease) in cash and cash equivalents	170,161	(504,242)
Cash and cash equivalents - beginning of period	1,342,257	2,111,084
Cash and cash equivalents - end of period	1,512,418	1,606,842
Supplemental cash flow information:
Taxes paid	77,278	103,965
Interest paid	24,630	24,630
[FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales	3,624,663	2,713,137
Purchases	(3,797,073)	(4,766,381)
[EquitySecuritiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales	428,226	457,170
Purchases	$ (471,158)	$ (330,323)