Segment Information (Summary Of Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 segments	Mar. 31, 2013
Segment Reporting Information [Line Items]
Gross premiums written	$ 901,393	$ 837,081
Net premiums written	771,614	695,052
Net premiums earned	530,285	463,228
Net losses and loss expenses	(275,286)	(255,178)
Acquisition costs	(67,722)	(56,685)
General and administrative expenses	(80,340)	(82,680)
Underwriting income (loss)	106,937	68,685
Net investment income	47,619	33,388
Net realized investment gains	54,205	79,637
Amortization of intangible assets	(633)	(633)
Interest expense	(14,534)	(14,134)
Foreign exchange loss	(49)	(2,518)
Income before income taxes	193,545	164,425
Loss and loss expense ratio	51.90%	55.10%
Acquisition cost ratio	12.80%	12.20%
General and administrative expense ratio	15.20%	17.80%
Combined ratio	79.90%	85.10%
Number of Operating Segments	3
Operating Segments [Member] | U S Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	269,945	256,018
Net premiums written	202,740	192,253
Net premiums earned	212,123	188,439
Net losses and loss expenses	(141,995)	(133,324)
Acquisition costs	(27,501)	(23,128)
General and administrative expenses	(37,436)	(39,596)
Underwriting income (loss)	5,191	(7,609)
Loss and loss expense ratio	66.90%	70.80%
Acquisition cost ratio	13.00%	12.30%
General and administrative expense ratio	17.60%	21.00%
Combined ratio	97.50%	104.10%
Operating Segments [Member] | International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	134,364	128,516
Net premiums written	75,459	77,745
Net premiums earned	88,339	84,214
Net losses and loss expenses	(23,566)	(28,935)
Acquisition costs	947	849
General and administrative expenses	(24,761)	(24,789)
Underwriting income (loss)	40,959	31,339
Loss and loss expense ratio	26.70%	34.40%
Acquisition cost ratio	(1.10%)	(1.00%)
General and administrative expense ratio	28.00%	29.40%
Combined ratio	53.60%	62.80%
Operating Segments [Member] | Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	497,084	452,547
Net premiums written	493,415	425,054
Net premiums earned	229,823	190,575
Net losses and loss expenses	(109,725)	(92,919)
Acquisition costs	(41,168)	(34,406)
General and administrative expenses	(18,143)	(18,295)
Underwriting income (loss)	$ 60,787	$ 44,955
Loss and loss expense ratio	47.70%	48.80%
Acquisition cost ratio	17.90%	18.10%
General and administrative expense ratio	7.90%	9.60%
Combined ratio	73.50%	76.50%