Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities
Net income	$ 248,099	$ 312,887
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of net premium on investments	29,911	29,366
Amortization of intangible assets	6,768	7,002
Net realized and unrealized investment gains	(115,645)	(142,172)
Changes in:
Reinsurance balances, net	(572,301)	(581,275)
Reinsurance recoverable on paid and unpaid losses, net of ceded premiums payable	45,691	86,002
Funds held by reinsured companies and funds held – directly managed	71,350	17,627
Deferred acquisition costs	(76,800)	(79,315)
Net tax assets and liabilities	37,445	(33,973)
Unpaid losses and loss expenses including life policy benefits	80,728	13,732
Unearned premiums	418,493	484,712
Other net changes in operating assets and liabilities	(34,992)	(35,852)
Net cash provided by operating activities	138,747	78,741
Payments/ proceeds from trading securities [Line Items]
Redemptions of fixed maturities	161,229	180,311
Sales and redemptions of short-term investments	12,692	15,830
Purchases of short-term investments	(7,391)	(31,141)
Other, net	(573)	13,295
Net cash provided by (used in) investing activities	114,775	(71,214)
Cash flows from financing activities
Dividends paid to common and preferred shareholders	(47,369)	(48,772)
Repurchase of common shares	(71,376)	(180,383)
Reissuance of treasury shares and issuance of common shares, net of taxes paid	(4,084)	(2,581)
Net cash used in financing activities	(122,829)	(231,736)
Effect of foreign exchange rate changes on cash	(30,362)	(3,239)
Increase (decrease) in cash and cash equivalents	100,331	(227,448)
Cash and cash equivalents—beginning of period	1,313,468	1,496,485
Cash and cash equivalents—end of period	1,413,799	1,269,037
Supplemental cash flow information:
Taxes paid	42,257	95,871
Interest paid	0	0
Fixed maturities [Member]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and sales of equities	2,495,633	1,734,246
Purchases of fixed maturities and purchases of equities	(2,293,577)	(1,963,428)
Equities [Member]
Payments/ proceeds from trading securities [Line Items]
Sales of fixed maturities and sales of equities	120,973	9,094
Purchases of fixed maturities and purchases of equities	$ (374,211)	$ (29,421)