Segment Information (Schedule of Segment Reporting Information, by Segment) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Gross premiums written	$ 519,598	$ 493,847	$ 1,080,286	$ 998,010
Net premiums written	395,803	369,795	876,674	803,087
Net premiums earned	355,307	338,924	690,183	677,248
Other income		616		913
Net losses and loss expenses	(235,813)	(188,722)	(540,265)	(420,876)
Acquisition costs	(42,971)	(37,938)	(81,053)	(78,722)
General and administrative expenses	(67,201)	(68,089)	(135,157)	(131,552)
Underwriting loss	9,322	44,791	(66,292)	47,011
Net investment income	52,368	65,594	102,576	134,496
Net realized investment gains	58,878	94,933	109,254	172,420
Net impairment charges recognized in earnings				(168)
Amortization and impairment of intangible assets	(766)	(891)	(1,533)	(1,783)
Interest expense	(13,745)	(9,531)	(27,487)	(19,059)
Foreign Currency Transaction Gain Loss Before Tax	(1,184)	(559)	(742)	(1,635)
Income before income taxes	104,873	194,337	115,776	331,282
Loss and loss expense ratio	66.40%	55.70%	78.30%	62.10%
Acquisition cost ratio	12.10%	11.20%	11.70%	11.60%
General and administrative expense ratio	18.90%	20.10%	19.60%	19.40%
Combined ratio	97.40%	87.00%	109.60%	93.10%
U.S.Insurance [Member]
Gross premiums written	226,738	189,663	410,040	351,748
Net premiums written	172,887	135,238	312,789	266,793
Net premiums earned	145,857	125,659	281,338	254,864
Other income		616		913
Net losses and loss expenses	(92,595)	(69,198)	(208,426)	(167,623)
Acquisition costs	(18,876)	(15,854)	(36,978)	(32,814)
General and administrative expenses	(31,253)	(30,683)	(62,052)	(57,797)
Underwriting loss	3,133	10,540	(26,118)	(2,457)
Net impairment charges recognized in earnings
Loss and loss expense ratio	63.50%	55.10%	74.10%	65.80%
Acquisition cost ratio	12.90%	12.60%	13.10%	12.90%
General and administrative expense ratio	21.40%	24.40%	22.10%	22.70%
Combined ratio	97.80%	92.10%	109.30%	101.40%
International Insurance [Member]
Gross premiums written	178,593	167,601	289,918	289,023
Net premiums written	108,985	98,509	183,895	179,590
Net premiums earned	79,956	89,427	156,246	176,470
Other income
Net losses and loss expenses	(72,082)	(64,580)	(143,266)	(122,029)
Acquisition costs	747	66	2,603
General and administrative expenses	(20,653)	(22,657)	(41,381)	(44,502)
Underwriting loss	(12,032)	2,256	(25,798)	9,939
Net impairment charges recognized in earnings
Loss and loss expense ratio	90.20%	72.20%	91.70%	69.10%
Acquisition cost ratio	(0.90%)	(0.10%)	(1.70%)	0.00%
General and administrative expense ratio	25.80%	25.30%	26.50%	25.20%
Combined ratio	115.10%	97.40%	116.50%	94.30%
Reinsurance [Member]
Gross premiums written	114,267	136,583	380,328	357,239
Net premiums written	113,931	136,048	379,990	356,704
Net premiums earned	129,494	123,838	252,599	245,914
Other income
Net losses and loss expenses	(71,136)	(54,944)	(188,573)	(131,224)
Acquisition costs	(24,842)	(22,150)	(46,678)	(45,908)
General and administrative expenses	(15,295)	(14,749)	(31,724)	(29,253)
Underwriting loss	18,221	31,995	(14,376)	39,529
Net impairment charges recognized in earnings
Loss and loss expense ratio	54.90%	44.40%	74.70%	53.40%
Acquisition cost ratio	19.20%	17.90%	18.50%	18.70%
General and administrative expense ratio	11.80%	11.90%	12.60%	11.90%
Combined ratio	85.90%	74.20%	105.80%	84.00%