Segment Information (Summary Of Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 segments	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Gross premiums written	$ 555,490	$ 580,893	$ 765,200	$ 837,081	$ 497,052	$ 504,420	$ 646,870	$ 680,929	$ 2,738,664	$ 2,329,271	$ 1,939,521
Net premiums written	391,130	453,077	581,222	695,052	362,623	391,537	494,710	588,953	2,120,481	1,837,823	1,533,766
Net premiums earned	524,561	510,773	507,271	463,228	476,244	441,017	429,747	401,890	2,005,833	1,748,898	1,456,992
Net losses and loss expenses	(315,966)	(276,970)	(275,128)	(255,178)	(414,734)	(258,948)	(240,380)	(225,202)	(1,123,242)	(1,139,264)	(959,156)
Acquisition costs	(66,257)	(65,114)	(64,617)	(56,685)	(55,910)	(51,086)	(51,588)	(47,138)	(252,673)	(205,722)	(167,295)
General and administrative expenses	(100,467)	(88,553)	(80,585)	(82,680)	(84,404)	(78,572)	(73,979)	(70,366)	(352,285)	(307,321)	(271,656)
Underwriting income (loss)									277,633	96,591	58,885
Net investment income	47,270	39,271	37,635	33,388	38,360	39,121	42,451	47,209	157,564	167,141	195,948
Net realized investment gains (losses)	67,599	27,487	(115,198)	79,637	14,379	149,813	8,663	133,581	59,525	306,436	10,077
Other income—termination fee											101,744
Amortization of intangible assets	(633)	(633)	(634)	(633)	(633)	(633)	(634)	(633)	(2,533)	(2,533)	(2,978)
Interest expense	(14,094)	(14,094)	(14,188)	(14,134)	(13,826)	(13,822)	(14,001)	(13,756)	(56,510)	(55,405)	(54,989)
Foreign exchange (loss) gain	(658)	(4,353)	(490)	(2,518)	(860)	(1,023)	1,019	81	(8,019)	(783)	(3,159)
Income before income taxes	141,355	127,814	(5,934)	164,425	(41,384)	225,867	101,298	225,666	427,660	511,447	305,528
Loss and loss expense ratio									56.00%	65.10%	65.80%
Acquisition cost ratio									12.60%	11.80%	11.50%
General and administrative expense ratio									17.60%	17.60%	18.60%
Combined ratio									86.20%	94.50%	95.90%
Number of Operating Segments									3
Operating Segments [Member] | U S Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written									1,163,146	993,918	838,567
Net premiums written									869,403	743,428	639,196
Net premiums earned									802,590	671,594	584,303
Net losses and loss expenses									(579,873)	(465,168)	(387,099)
Acquisition costs									(106,704)	(86,670)	(74,976)
General and administrative expenses									(165,482)	(141,729)	(124,434)
Underwriting income (loss)									(49,469)	(21,973)	(2,206)
Loss and loss expense ratio									72.30%	69.30%	66.20%
Acquisition cost ratio									13.30%	12.90%	12.80%
General and administrative expense ratio									20.60%	21.10%	21.30%
Combined ratio									106.20%	103.30%	100.30%
Operating Segments [Member] | International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written									641,764	575,103	530,450
Net premiums written									358,040	346,348	325,094
Net premiums earned									346,397	336,787	317,006
Net losses and loss expenses									(121,789)	(163,130)	(206,593)
Acquisition costs									1,784	486	2,781
General and administrative expenses									(106,724)	(91,867)	(84,290)
Underwriting income (loss)									119,668	82,276	28,904
Loss and loss expense ratio									35.20%	48.40%	65.20%
Acquisition cost ratio									(0.50%)	(0.10%)	(0.90%)
General and administrative expense ratio									30.80%	27.30%	26.60%
Combined ratio									65.50%	75.60%	90.90%
Operating Segments [Member] | Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written									933,754	760,250	570,504
Net premiums written									893,038	748,047	569,476
Net premiums earned									856,846	740,517	555,683
Net losses and loss expenses									(421,580)	(510,966)	(365,464)
Acquisition costs									(147,753)	(119,538)	(95,100)
General and administrative expenses									(80,079)	(73,725)	(62,932)
Underwriting income (loss)									$ 207,434	$ 36,288	$ 32,187
Loss and loss expense ratio									49.20%	69.00%	65.80%
Acquisition cost ratio									17.20%	16.10%	17.10%
General and administrative expense ratio									9.30%	10.00%	11.30%
Combined ratio									75.70%	95.10%	94.20%