Unaudited Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Cash flows from operating activities
Net income (loss)	$ 360,141	$ (806,956)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Amortization of net premium on investments	30,216	17,759
Amortization of intangible assets	8,893	8,827
Net realized and unrealized investment (gains) losses	(192,735)	112,199
Changes in:
Reinsurance balances, net	(442,507)	(404,556)
Reinsurance recoverable on paid and unpaid losses, net of ceded premiums payable	75,219	19,179
Funds held by reinsured companies and funds held - directly managed	42,851	393,061
Deferred acquisition costs	(78,286)	(51,434)
Net tax assets and liabilities	25,952	(68,732)
Unpaid losses and loss expenses including life policy benefits	(257,827)	821,108
Unearned premiums	483,456	405,830
Other net changes in operating assets and liabilities	23,650	40,029
Net cash provided by operating activities	79,023	486,314
Cash flows from investing activities
Redemptions Of Fixed Maturities	311,062	429,158
Payments/ proceeds from trading securities [Line Items]
Sales and redemptions of short-term investments	36,132	26,787
Purchases of short-term investments	(32,168)	(55,797)
Other, net	41,224	20,753
Net cash used in investing activities	(167,693)	(319,830)
Cash flows from financing activities
Cash dividends paid to shareholders	(55,948)	(45,776)
Repurchase of common shares	0	(244,222)
Issuance of common shares	5,601	3,616
Net cash used in financing activities	(50,347)	(286,382)
Effect of foreign exchange rate changes on cash	6,989	18,551
Decrease in cash and cash equivalents	(132,028)	(101,347)
Cash and cash equivalents - beginning of period	1,342,257	2,111,084
Cash and cash equivalents - end of period	1,210,229	2,009,737
Supplemental cash flow information:
Taxes paid	42,386	42,177
Interest paid	0	0
[FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales	1,621,825	839,611
Purchases	(2,241,991)	(1,553,630)
[EquitySecuritiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales	332,851	154,861
Purchases	$ (236,628)	$ (181,573)