Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities
Net income (loss)	$ 1,134,514	$ (520,291)	$ 852,552
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Amortization of net premium on investments	137,313	91,339	78,620
Amortization of intangible assets	31,799	36,405	31,461
Net realized and unrealized investment gains	(493,409)	(66,692)	(401,482)
Changes in:
Reinsurance balances, net	(102,009)	(21,036)	217,066
Reinsurance recoverable on paid and unpaid losses, net of ceded premiums payable	31,730	625	(30,033)
Funds held by reinsured companies and funds held - directly managed	381,733	606,266	296,174
Deferred acquisition costs	(13,437)	52,069	35,317
Net tax assets and liabilities	80,628	(58,970)	(56,599)
Unpaid losses and loss expenses including life policy benefits	(634,736)	606,698	227,240
Unearned premiums	86,921	(161,425)	(71,355)
Other, net	52,246	8,647	47,959
Net cash provided by operating activities	693,293	573,635	1,226,920
Cash flows from investing activities
Redemptions of fixed maturities	1,000,181	1,211,016	1,272,885
Payments/ proceeds from trading securities [Line Items]
Sales and redemptions of short-term investments	110,360	336,456	270,087
Purchases of short-term investments	(215,473)	(331,432)	(141,157)
Other, net	995	(186,823)	(185,965)
Consideration paid, net of cash acquired in a business combination	(9,242)	0	0
Net cash (used in) provided by investing activities	(219,538)	(1,080,378)	1,102,508
Cash flows from financing activities
Cash dividends paid to shareholders	(217,615)	(205,784)	(192,156)
Repurchase of common shares	(504,991)	(413,737)	(1,065,121)
Issuance of common shares	34,323	16,041	37,682
Net proceeds from issuance of preferred shares	0	361,722	0
Proceeds from issuance of senior notes	0	0	500,000
Contract fees on forward sale agreement	0	0	(2,638)
Repayment of debt	0	0	(200,000)
Net cash used in financing activities	(688,283)	(241,758)	(922,233)
Effect of foreign exchange rate changes on cash	(6,024)	(20,326)	(34,420)
(Decrease) increase in cash and cash equivalents	(220,552)	(768,827)	1,372,775
Cash and cash equivalents - beginning of year	1,342,257	2,111,084	738,309
Cash and cash equivalents - end of year	1,121,705	1,342,257	2,111,084
Supplemental cash flow information:
Taxes paid	186,970	197,610	199,838
Interest paid	49,259	49,259	42,995
[FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	6,969,074	8,328,352	8,621,227
Purchases of fixed maturities and equities	(8,067,087)	(10,549,343)	(8,572,471)
[EquitySecuritiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	821,977	730,929	607,459
Purchases of fixed maturities and equities	$ (830,323)	$ (619,533)	$ (769,557)