Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Cash flows from operating activities
Net income	$ 59,982	$ 536,287
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of net premium on investments	82,147	64,054
Amortization of intangible assets	14,091	17,786
Net realized and unrealized investment (losses) gains	276,272	(230,867)
Changes in:
Reinsurance balances, net	(555,356)	(281,652)
Reinsurance recoverable on paid and unpaid losses, net of ceded premiums payable	101,696	33,925
Funds held by reinsured companies and funds held - directly managed	62,440	36,609
Deferred acquisition costs	(121,359)	(81,990)
Net tax assets and liabilities	(144,342)	50,431
Unpaid losses and loss expenses including life policy benefits	(181,198)	(494,083)
Unearned premiums	590,434	528,623
Other net changes in operating assets and liabilities	45,951	3,445
Net cash provided by operating activities	230,758	182,568
Cash flows from investing activities
Redemptions of fixed maturities	772,227	512,544
Payments/ proceeds from trading securities [Line Items]
Sales and redemptions of short-term investments	226,390	52,804
Purchases of short-term investments	(105,446)	(42,046)
Other, net	(7,122)	16,116
Net cash provided by (used in) investing activities	489,032	324,076
Cash flows from financing activities
Dividends paid to shareholders	(103,294)	(110,735)
Repurchase of common shares	(496,023)	(221,995)
Issuance of common shares	34,416	16,036
Net proceeds from issuance of preferred shares	241,265	0
Repurchase of preferred shares	(290,000)	0
Sale of shares to noncontrolling interests	47,136	0
Net cash used in financing activities	(566,500)	(316,694)
Effect of foreign exchange rate changes on cash	(13,455)	(19,789)
Increase (decrease) in cash and cash equivalents	139,835	170,161
Cash and cash equivalents - beginning of period	1,121,705	1,342,257
Cash and cash equivalents - end of period	1,261,540	1,512,418
Supplemental cash flow information:
Taxes paid	112,671	77,278
Interest paid	24,630	24,630
[FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	3,844,517	3,624,663
Purchases of fixed maturities and equities	(4,198,801)	(3,797,073)
[EquitySecuritiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	539,498	428,226
Purchases of fixed maturities and equities	$ (582,231)	$ (471,158)