Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities
Net income	$ 397,517	$ 1,022,994
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of net premium on investments	119,468	99,722
Amortization of intangible assets	21,136	26,679
Net realized and unrealized investment losses (gains)	260,154	(488,296)
Changes in:
Reinsurance balances, net	(592,380)	(202,270)
Reinsurance recoverable on paid and unpaid losses, net of ceded premiums payable	65,593	59,536
Funds held by reinsured companies and funds held - directly managed	76,159	110,096
Deferred acquisition costs	(111,562)	(51,722)
Net tax assets and liabilities	(83,379)	89,900
Unpaid losses and loss expenses including life policy benefits	(65,725)	(557,411)
Unearned premiums	433,740	334,772
Other net changes in operating assets and liabilities	63,014	25,355
Net cash provided by operating activities	583,735	469,355
Cash flows from investing activities
Redemptions of fixed maturities	1,002,991	705,423
Payments/ proceeds from trading securities [Line Items]
Sales and redemptions of short-term investments	290,011	61,903
Purchases of short-term investments	(176,339)	(176,068)
Other, net	98,813	33,891
Net cash provided by (used in) investing activities	584,512	(11,210)
Cash flows from financing activities
Dividends paid to shareholders	(151,869)	(164,368)
Repurchase of common shares	(619,534)	(312,291)
Issuance of common shares	37,193	19,117
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	(735,809)	(457,542)
Effect of foreign exchange rate changes on cash	(3,081)	(6,498)
Increase (decrease) in cash and cash equivalents	429,357	(5,895)
Cash and cash equivalents - beginning of period	1,121,705	1,342,257
Cash and cash equivalents - end of period	1,551,062	1,336,362
Supplemental cash flow information:
Taxes paid	148,522	105,059
Interest paid	24,630	24,630
[FixedMaturitiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	5,831,364	5,016,192
Purchases of fixed maturities and equities	(6,501,873)	(5,649,511)
[EquitySecuritiesMember]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and equities	595,848	574,612
Purchases of fixed maturities and equities	$ (556,303)	$ (577,652)