Segment Information (Summary Of Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 segments	Sep. 30, 2013
Segment Reporting Information [Line Items]
Gross premiums written	$ 707,884	$ 580,893	$ 2,369,682	$ 2,183,174
Net premiums written	568,742	453,077	1,894,280	1,729,351
Net premiums earned	541,737	510,773	1,609,269	1,481,272
Net losses and loss expenses	(336,090)	(276,970)	(926,231)	(807,276)
Acquisition costs	(72,403)	(65,114)	(214,404)	(186,416)
General and administrative expenses	(88,294)	(88,553)	(264,822)	(251,818)
Underwriting (loss) income	44,950	80,136	203,812	235,762
Other insurance-related income	1,032	0	1,032	0
Other insurance-related expenses	(6,575)	0	(6,575)	0
Segment (loss) income	39,407	80,136	198,269	235,762
Net investment income	43,412	39,271	127,824	110,294
Net realized investment (losses) gains	(35,136)	27,487	104,286	(8,074)
Amortization of intangible assets	(633)	(633)	(1,900)	(1,900)
Interest expense	(14,325)	(14,094)	(43,451)	(42,416)
Foreign exchange loss	(278)	(4,353)	(978)	(7,361)
Income before income taxes	32,447	127,814	384,050	286,305
Loss and loss expense ratio	62.00%	54.20%	57.60%	54.50%
Acquisition cost ratio	13.40%	12.70%	13.30%	12.60%
General and administrative expense ratio	16.30%	17.30%	16.50%	17.00%
Expense ratio	29.70%	30.00%	29.80%	29.60%
Combined ratio	91.70%	84.20%	87.40%	84.10%
Number of operating segments			3
Operating Segments [Member] | U S Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	386,681	308,709	998,051	872,024
Net premiums written	321,713	238,792	746,403	652,464
Net premiums earned	224,764	207,602	651,480	593,477
Net losses and loss expenses	(153,010)	(141,222)	(440,491)	(398,910)
Acquisition costs	(31,131)	(28,426)	(88,311)	(78,824)
General and administrative expenses	(41,730)	(41,616)	(125,760)	(119,514)
Underwriting (loss) income	(1,107)	(3,662)	(3,082)	(3,771)
Other insurance-related income	1,032	0	1,032	0
Other insurance-related expenses	(1,270)	0	(1,270)	0
Segment (loss) income	(1,345)	(3,662)	(3,320)	(3,771)
Loss and loss expense ratio	68.10%	68.00%	67.60%	67.20%
Acquisition cost ratio	13.90%	13.70%	13.60%	13.30%
General and administrative expense ratio	18.60%	20.00%	19.30%	20.10%
Expense ratio	32.50%	33.70%	32.90%	33.40%
Combined ratio	100.60%	101.70%	100.50%	100.60%
Operating Segments [Member] | International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	144,236	132,881	483,079	453,990
Net premiums written	87,820	75,632	285,450	259,771
Net premiums earned	94,013	87,554	271,557	258,809
Net losses and loss expenses	(55,814)	(31,094)	(115,299)	(90,997)
Acquisition costs	(1,209)	282	(835)	1,489
General and administrative expenses	(27,993)	(26,450)	(82,164)	(75,374)
Underwriting (loss) income	8,997	30,292	73,259	93,927
Other insurance-related income	0	0	0	0
Other insurance-related expenses	(5,305)	0	(5,305)	0
Segment (loss) income	3,692	30,292	67,954	93,927
Loss and loss expense ratio	59.40%	35.50%	42.50%	35.20%
Acquisition cost ratio	1.30%	(0.30%)	0.30%	(0.60%)
General and administrative expense ratio	29.80%	30.20%	30.30%	29.10%
Expense ratio	31.10%	29.90%	30.60%	28.50%
Combined ratio	90.50%	65.40%	73.10%	63.70%
Operating Segments [Member] | Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	176,967	139,303	888,552	857,160
Net premiums written	159,209	138,653	862,427	817,116
Net premiums earned	222,960	215,617	686,232	628,986
Net losses and loss expenses	(127,266)	(104,654)	(370,441)	(317,369)
Acquisition costs	(40,063)	(36,970)	(125,258)	(109,081)
General and administrative expenses	(18,571)	(20,487)	(56,898)	(56,930)
Underwriting (loss) income	37,060	53,506	133,635	145,606
Other insurance-related income	0	0	0	0
Other insurance-related expenses	0	0	0	0
Segment (loss) income	$ 37,060	$ 53,506	$ 133,635	$ 145,606
Loss and loss expense ratio	57.10%	48.50%	54.00%	50.50%
Acquisition cost ratio	18.00%	17.10%	18.30%	17.30%
General and administrative expense ratio	8.30%	9.50%	8.30%	9.10%
Expense ratio	26.30%	26.60%	26.60%	26.40%
Combined ratio	83.40%	75.10%	80.60%	76.90%