Segment Information (Schedule Of Segment Reporting Information, By Segment) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Gross premiums written	$ 442,698	$ 378,445	$ 1,522,984	$ 1,376,455
Net premiums written	350,260	302,169	1,226,934	1,105,256
Net premiums earned	371,340	339,496	1,061,523	1,016,744
Other income				913
Net losses and loss expenses	(205,546)	(126,988)	(745,811)	(547,864)
Acquisition costs	(39,680)	(41,919)	(120,733)	(120,641)
General and administrative expenses	(66,007)	(69,871)	(201,164)	(201,423)
Underwriting (loss) income	60,107	100,718	(6,185)	147,729
Net investment income	47,883	59,479	150,459	193,975
Net realized investment gains (losses)	(130,809)	116,930	(21,555)	289,350
Other income - termination fee	35,000		35,000
Net impairment charges recognized in earnings				(168)
Amortization and impairment of intangible assets	(767)	(892)	(2,300)	(2,675)
Interest expense	(13,748)	(9,533)	(41,235)	(28,592)
Foreign exchange loss (gain)	(2,966)	1,387	(3,708)	(248)
(Loss) Income before income taxes	(5,300)	268,089	110,476	599,371
Loss and loss expense ratio	55.40%	37.40%	70.30%	53.90%
Acquisition cost ratio	10.70%	12.30%	11.40%	11.90%
General and administrative expense ratio	17.80%	20.60%	19.00%	19.80%
Combined ratio	83.90%	70.30%	100.70%	85.60%
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	201,522	181,232	611,562	532,980
Net premiums written	157,310	140,481	470,099	407,274
Net premiums earned	150,474	129,650	431,812	384,514
Other income				913
Net losses and loss expenses	(85,720)	(55,144)	(294,146)	(222,767)
Acquisition costs	(19,549)	(18,081)	(56,527)	(50,895)
General and administrative expenses	(28,945)	(31,781)	(90,997)	(89,578)
Underwriting (loss) income	16,260	24,644	(9,858)	22,187
Net impairment charges recognized in earnings
Loss and loss expense ratio	57.00%	42.50%	68.10%	57.90%
Acquisition cost ratio	13.00%	13.90%	13.10%	13.20%
General and administrative expense ratio	19.20%	24.50%	21.10%	23.30%
Combined ratio	89.20%	80.90%	102.30%	94.40%
International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	109,612	100,858	399,530	389,881
Net premiums written	61,386	65,520	245,281	245,110
Net premiums earned	80,175	80,557	236,421	257,027
Other income
Net losses and loss expenses	(43,666)	(11,040)	(186,932)	(133,069)
Acquisition costs	343	29	2,946	29
General and administrative expenses	(21,558)	(22,819)	(62,939)	(67,321)
Underwriting (loss) income	15,294	46,727	(10,504)	56,666
Net impairment charges recognized in earnings
Loss and loss expense ratio	54.50%	13.70%	79.10%	51.80%
Acquisition cost ratio	(0.40%)	0.00%	(1.20%)	0.00%
General and administrative expense ratio	26.90%	28.30%	26.60%	26.20%
Combined ratio	81.00%	42.00%	104.50%	78.00%
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	131,564	96,355	511,892	453,594
Net premiums written	131,564	96,168	511,554	452,872
Net premiums earned	140,691	129,289	393,290	375,203
Other income
Net losses and loss expenses	(76,160)	(60,804)	(264,733)	(192,028)
Acquisition costs	(20,474)	(23,867)	(67,152)	(69,775)
General and administrative expenses	(15,504)	(15,271)	(47,228)	(44,524)
Underwriting (loss) income	28,553	29,347	14,177	68,876
Net impairment charges recognized in earnings
Loss and loss expense ratio	54.10%	47.00%	67.30%	51.20%
Acquisition cost ratio	14.60%	18.50%	17.10%	18.60%
General and administrative expense ratio	11.00%	11.80%	12.00%	11.90%
Combined ratio	79.70%	77.30%	96.40%	81.70%