Segment Information (Summary Of Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Gross premiums written	$ 416,537	$ 442,698	$ 519,598	$ 560,688	$ 381,942	$ 378,445	$ 493,847	$ 504,163	$ 1,939,521	$ 1,758,397	$ 1,696,345
Net premiums written	306,832	350,260	395,803	480,871	287,199	302,169	369,795	433,292	1,533,766	1,392,455	1,321,125
Net premiums earned	395,469	371,340	355,307	334,876	342,804	339,496	338,924	338,324	1,456,992	1,359,548	1,316,892
Other income							616	297	101,744	913	1,506
Net losses and loss expenses	(213,345)	(205,546)	(235,813)	(304,452)	(160,019)	(126,988)	(188,722)	(232,154)	(959,156)	(707,883)	(604,060)
Acquisition costs	(46,562)	(39,680)	(42,971)	(38,082)	(38,848)	(41,919)	(37,938)	(40,784)	(167,295)	(159,489)	(148,847)
General and administrative expenses	(70,492)	(66,007)	(67,201)	(67,956)	(85,134)	(69,871)	(68,089)	(63,463)	(271,656)	(286,557)	(248,592)
Underwriting (loss) income									58,885	206,532	316,899
Net investment income	45,489	47,883	52,368	50,208	50,168	59,479	65,594	68,902	195,948	244,143	300,675
Net realized investment gains (losses)	31,632	(130,809)	58,878	50,376	(3,738)	116,930	94,933	77,487	10,077	285,612	126,352
Other income - termination fee	66,744	35,000							101,744
Net impairment charges recognized in earnings								168		(168)	(49,577)
Amortization and impairment of intangible assets	(678)	(767)	(766)	(767)	(808)	(892)	(891)	(892)	(2,978)	(3,483)	(11,051)
Interest expense	(13,754)	(13,748)	(13,745)	(13,742)	(11,650)	(9,533)	(9,531)	(9,528)	(54,989)	(40,242)	(39,019)
Foreign exchange loss	549	(2,966)	(1,184)	442	(196)	1,387	(559)	(1,076)	(3,159)	(444)	(748)
Income before income taxes	195,052	(5,300)	104,873	10,903	92,579	268,089	194,337	136,945	305,528	691,950	643,531
Loss and loss expense ratio									65.80%	52.10%	45.90%
Acquisition cost ratio									11.50%	11.70%	11.30%
General and administrative expense ratio									18.60%	21.10%	18.90%
Combined ratio									95.90%	84.90%	76.10%
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written									838,567	729,267	674,826
Net premiums written									639,196	551,063	493,067
Net premiums earned									584,303	518,444	447,491
Other income										913	1,506
Net losses and loss expenses									(387,099)	(297,517)	(211,363)
Acquisition costs									(74,976)	(67,797)	(58,114)
General and administrative expenses									(124,434)	(128,556)	(115,797)
Underwriting (loss) income									(2,206)	25,487	63,723
Loss and loss expense ratio									66.20%	57.40%	47.20%
Acquisition cost ratio									12.80%	13.10%	13.00%
General and administrative expense ratio									21.30%	24.80%	25.90%
Combined ratio									100.30%	95.30%	86.10%
International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written									530,450	504,937	555,944
Net premiums written									325,094	319,083	362,893
Net premiums earned									317,006	338,791	413,170
Net losses and loss expenses									(206,593)	(160,153)	(158,062)
Acquisition costs									2,781	460	(2,742)
General and administrative expenses									(84,290)	(94,226)	(84,390)
Underwriting (loss) income									28,904	84,872	167,976
Loss and loss expense ratio									65.20%	47.30%	38.30%
Acquisition cost ratio									(0.90%)	(0.10%)	0.70%
General and administrative expense ratio									26.60%	27.80%	20.40%
Combined ratio									90.90%	75.00%	59.40%
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written									570,504	524,193	465,575
Net premiums written									569,476	522,309	465,165
Net premiums earned									555,683	502,313	456,231
Net losses and loss expenses									(365,464)	(250,213)	(234,635)
Acquisition costs									(95,100)	(92,152)	(87,991)
General and administrative expenses									(62,932)	(63,775)	(48,405)
Underwriting (loss) income									$ 32,187	$ 96,173	$ 85,200
Loss and loss expense ratio									65.80%	49.80%	51.40%
Acquisition cost ratio									17.10%	18.30%	19.30%
General and administrative expense ratio									11.30%	12.70%	10.60%
Combined ratio									94.20%	80.80%	81.30%