Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ (80,486)	$ 610,754	$ 806,989
Adjustments to reconcile net income to net cash provided by operating activities:
Decrease (increase) in premiums receivable	(235,560)	132,986	(52,966)
Decrease (increase) in funds held by reinsureds, net	18,236	1,573	(25,271)
Decrease (increase) in reinsurance receivables	97,549	(62,954)	54,674
Decrease (increase) in deferred tax asset	(194,029)	22,023	147,071
Decrease (increase) in prepaid reinsurance premiums	46,374	(25,281)	(24,161)
Increase (decrease) in reserve for losses and loss adjustment expenses	826,230	420,748	(66,177)
Increase (decrease) in future policy benefit reserve	4,185	(1,534)	(1,636)
Increase (decrease) in unearned premiums	(39,822)	36,883	64,892
Change in equity adjustments in limited partnerships	(56,549)	(71,493)	20,575
Change in other assets and liabilities, net	214,701	(104,276)	(109,004)
Non-cash compensation expense	17,693	14,786	13,347
Amortization of bond premium (accrual of bond discount)	47,872	46,095	32,172
Amortization of underwriting discount on senior notes	49	76	192
Realized gain on debt repurchase			(78,271)
Net realized capital (gains) losses	(6,923)	(101,911)	2,312
Net cash provided by (used in) operating activities	659,520	918,475	784,738
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from fixed maturities matured/called - available for sale, at market value	1,702,973	1,717,659	1,203,548
Proceeds from fixed maturities matured/called - available for sale, at fair value	12,775		15,358
Proceeds from fixed maturities sold - available for sale, at market value	1,732,246	1,632,719	311,273
Proceeds from fixed maturities sold - available for sale, at fair value	65,158	20,237	14,777
Proceeds from equity securities sold - available for sale, at market value	27,207	3,037	24,159
Proceeds from equity securities sold - available for sale, at fair value	247,227	234,112	43,496
Distributions from other invested assets	166,273	79,849	182,952
Cost of fixed maturities acquired - available for sale, at market value	(3,238,113)	(2,766,212)	(3,051,012)
Cost of fixed maturities acquired - available for sale, at fair value	(27,481)	(134,324)	(27,555)
Cost of equity securities acquired - available for sale, at market value	(127,837)	(353,265)
Cost of equity securities acquired - available for sale, at fair value	(755,734)	(514,092)	(265,275)
Cost of other invested assets acquired	(64,832)	(66,408)	(62,554)
Cost of businesses acquired	(63,100)
Net change in short-term investments	100,969	(110,241)	1,228,032
Net change in unsettled securities transactions	13,563	(13,084)	10,445
Net cash provided by (used in) investing activities	(208,706)	(270,013)	(372,356)
CASH FLOWS FROM FINANCING ACTIVITIES:
Common shares issued during the period, net	12,269	3,066	7,284
Purchase of treasury shares	(92,490)	(398,554)	(190,597)
Revolving credit borrowings	(50,000)	50,000
Net cost of debt repurchase			(83,026)
Net cost of senior notes maturing		(200,000)
Dividends paid to shareholders	(103,848)	(108,477)	(116,857)
Net cash provided by (used in) financing activities	(234,069)	(653,965)	(383,196)
EFFECT OF EXCHANGE RATE CHANGES ON CASH	(26,502)	16,313	12,718
Net increase (decrease) in cash	190,243	10,810	41,904
Cash, beginning of period	258,408	247,598	205,694
Cash, end of period	448,651	258,408	247,598
SUPPLEMENTAL CASH FLOW INFORMATION:
Income taxes paid (recovered)	(44,537)	(30,978)	111,831
Interest paid	51,647	60,198	72,454
Non-cash transaction:
Net assets acquired and liabilities assumed from business acquisitions	$ 19,130