Segment Information (Summary Of Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Gross premiums written	$ 765,200	$ 646,870	$ 1,602,281	$ 1,327,799
Net premiums written	581,222	494,710	1,276,274	1,083,663
Net premiums earned	507,271	429,747	970,499	831,637
Net losses and loss expenses	(275,128)	(240,380)	(530,306)	(465,582)
Acquisition costs	(64,617)	(51,588)	(121,302)	(98,726)
General and administrative expenses	(80,585)	(73,979)	(163,265)	(144,345)
Underwriting income (loss)	86,941	63,800	155,626	122,984
Net investment income	37,635	42,451	71,023	89,660
Net realized investment (losses) gains	(115,198)	8,663	(35,561)	142,244
Amortization of intangible assets	(634)	(634)	(1,267)	(1,267)
Interest expense	(14,188)	(14,001)	(28,322)	(27,757)
Foreign exchange (loss) gain	(490)	1,019	(3,008)	1,100
Loss (income) before income taxes	(5,934)	101,298	158,491	326,964
Loss and loss expense ratio	54.20%	55.90%	54.60%	56.00%
Acquisition cost ratio	12.70%	12.00%	12.50%	11.90%
General and administrative expense ratio	15.90%	17.20%	16.80%	17.30%
Combined ratio	82.80%	85.10%	83.90%	85.20%
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	307,297	265,974	563,315	470,185
Net premiums written	221,419	196,661	413,672	350,507
Net premiums earned	197,436	162,785	385,875	316,143
Net losses and loss expenses	(124,364)	(103,074)	(257,688)	(200,778)
Acquisition costs	(27,270)	(21,250)	(50,398)	(41,222)
General and administrative expenses	(38,302)	(34,730)	(77,898)	(65,774)
Underwriting income (loss)	7,500	3,731	(109)	8,369
Loss and loss expense ratio	63.00%	63.30%	66.80%	63.50%
Acquisition cost ratio	13.80%	13.10%	13.10%	13.00%
General and administrative expense ratio	19.40%	21.30%	20.20%	20.80%
Combined ratio	96.20%	97.70%	100.10%	97.30%
International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	192,593	183,593	321,109	297,183
Net premiums written	106,394	111,342	184,139	183,951
Net premiums earned	87,041	82,605	171,255	162,476
Net losses and loss expenses	(30,968)	(22,233)	(59,903)	(60,333)
Acquisition costs	358	582	1,207	1,110
General and administrative expenses	(24,135)	(21,648)	(48,924)	(44,049)
Underwriting income (loss)	32,296	39,306	63,635	59,204
Loss and loss expense ratio	35.60%	26.90%	35.00%	37.10%
Acquisition cost ratio	(0.40%)	(0.70%)	(0.70%)	(0.70%)
General and administrative expense ratio	27.70%	26.20%	28.60%	27.10%
Combined ratio	62.90%	52.40%	62.90%	63.50%
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	265,310	197,303	717,857	560,431
Net premiums written	253,409	186,707	678,463	549,205
Net premiums earned	222,794	184,357	413,369	353,018
Net losses and loss expenses	(119,796)	(115,073)	(212,715)	(204,471)
Acquisition costs	(37,705)	(30,920)	(72,111)	(58,614)
General and administrative expenses	(18,148)	(17,601)	(36,443)	(34,522)
Underwriting income (loss)	$ 47,145	$ 20,763	$ 92,100	$ 55,411
Loss and loss expense ratio	53.80%	62.40%	51.50%	57.90%
Acquisition cost ratio	16.90%	16.80%	17.40%	16.60%
General and administrative expense ratio	8.10%	9.50%	8.80%	9.80%
Combined ratio	78.80%	88.70%	77.70%	84.30%