Segment Information (Summary Of Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 segments	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross premiums written	$ 565,743	$ 707,884	$ 760,405	$ 901,393	$ 555,490	$ 580,893	$ 765,200	$ 837,081	$ 2,935,425	$ 2,738,664	$ 2,329,271
Net premiums written	427,795	568,742	553,924	771,614	391,130	453,077	581,222	695,052	2,322,075	2,120,481	1,837,823
Net premiums earned	573,489	541,737	537,247	530,285	524,561	510,773	507,271	463,228	2,182,758	2,005,833	1,748,898
Net losses and loss expenses	(272,959)	(336,090)	(314,855)	(275,286)	(315,966)	(276,970)	(275,128)	(255,178)	(1,199,190)	(1,123,242)	(1,139,264)
Acquisition costs	(80,718)	(72,403)	(74,279)	(67,722)	(66,257)	(65,114)	(64,617)	(56,685)	(295,122)	(252,673)	(205,722)
General and administrative expenses	(100,914)	(88,294)	(96,188)	(80,340)	(100,467)	(88,553)	(80,585)	(82,680)	(365,736)	(352,285)	(307,321)
Underwriting income									322,710	277,633	96,591
Other insurance-related revenue	1,032	1,032	0	0					2,064	0	0
Other insurance-related expenses	(2,003)	(6,575)	0	0					(8,578)	0	0
Segment income									316,196	277,633	96,591
Net investment income	49,050	43,412	36,793	47,619	47,270	39,271	37,635	33,388	176,874	157,564	167,141
Net realized investment gains (losses)	(15,329)	(35,136)	85,217	54,205	67,599	27,487	(115,198)	79,637	88,957	59,525	306,436
Amortization of intangible assets	(633)	(633)	(634)	(633)	(633)	(633)	(634)	(633)	(2,533)	(2,533)	(2,533)
Interest expense	(14,304)	(14,325)	(14,592)	(14,534)	(14,094)	(14,094)	(14,188)	(14,134)	(57,755)	(56,510)	(55,405)
Foreign exchange (loss) gain	21	(278)	(651)	(49)	(658)	(4,353)	(490)	(2,518)	(957)	(8,019)	(783)
Income before income taxes	136,732	32,447	158,058	193,545	141,355	127,814	(5,934)	164,425	520,782	427,660	511,447
Loss and loss expense ratio									54.90%	56.00%	65.10%
Acquisition cost ratio									13.50%	12.60%	11.80%
General and administrative expense ratio									16.80%	17.60%	17.60%
Expense ratio									30.30%	30.20%	29.40%
Combined ratio									85.20%	86.20%	94.50%
Number of operating segments									3
Operating segments | North American Insurance
Segment Reporting Information [Line Items]
Gross premiums written									1,716,288	1,572,355	1,376,040
Net premiums written									1,230,839	1,082,441	964,199
Net premiums earned									1,111,164	1,022,979	892,721
Net losses and loss expenses									(683,836)	(651,267)	(608,762)
Acquisition costs									(105,914)	(94,869)	(77,701)
General and administrative expenses									(219,665)	(209,042)	(185,231)
Underwriting income									101,749	67,801	21,027
Other insurance-related revenue									2,064	0	0
Other insurance-related expenses									(1,854)	0	0
Segment income									101,959	67,801	21,027
Loss and loss expense ratio									61.50%	63.70%	68.20%
Acquisition cost ratio									9.50%	9.30%	8.70%
General and administrative expense ratio									19.80%	20.40%	20.70%
Expense ratio									29.30%	29.70%	29.40%
Combined ratio									90.80%	93.40%	97.60%
Operating segments | Global Markets Insurance
Segment Reporting Information [Line Items]
Gross premiums written									280,543	232,555	192,981
Net premiums written									187,997	145,002	125,577
Net premiums earned									162,566	126,008	115,660
Net losses and loss expenses									(61,057)	(50,395)	(19,536)
Acquisition costs									(18,222)	(10,051)	(8,483)
General and administrative expenses									(68,106)	(63,164)	(48,365)
Underwriting income									15,181	2,398	39,276
Other insurance-related revenue									0	0	0
Other insurance-related expenses									(6,724)	0	0
Segment income									8,457	2,398	39,276
Loss and loss expense ratio									37.60%	40.00%	16.90%
Acquisition cost ratio									11.20%	8.00%	7.30%
General and administrative expense ratio									41.90%	50.10%	41.80%
Expense ratio									53.10%	58.10%	49.10%
Combined ratio									90.70%	98.10%	66.00%
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written									938,594	933,754	760,250
Net premiums written									903,239	893,038	748,047
Net premiums earned									909,028	856,846	740,517
Net losses and loss expenses									(454,297)	(421,580)	(510,966)
Acquisition costs									(170,986)	(147,753)	(119,538)
General and administrative expenses									(77,965)	(80,079)	(73,725)
Underwriting income									205,780	207,434	36,288
Other insurance-related revenue									0	0	0
Other insurance-related expenses									0	0	0
Segment income									$ 205,780	$ 207,434	$ 36,288
Loss and loss expense ratio									50.00%	49.20%	69.00%
Acquisition cost ratio									18.80%	17.20%	16.10%
General and administrative expense ratio									8.60%	9.30%	10.00%
Expense ratio									27.40%	26.50%	26.10%
Combined ratio									77.40%	75.70%	95.10%