SEGMENT INFORMATION (DETAILS) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Segment Information [Line Items]
Gross premiums written	$ 835,056	$ 750,687	$ 3,429,649	$ 3,115,761
Net premiums written	673,453	626,322	2,924,372	2,658,896
Net premiums earned	839,992	758,873	2,468,207	2,190,092
Other insurance related income	1,156	884	2,047	1,727
Net losses and loss expenses	(506,839)	(422,154)	(2,091,598)	(1,293,787)
Acquisition costs	(146,836)	(123,788)	(430,097)	(364,614)
General and administrative expenses	(97,444)	(86,439)	(290,860)	(256,762)
Underwriting income (loss)	90,029	127,376	(342,301)	276,656
Corporate expenses	(17,093)	(16,996)	(58,302)	(52,504)
Net investment income	49,396	111,800	260,068	299,004
Net realized investment gains	57,557	76,531	125,177	117,325
Foreign exchange (losses) gains	60,830	(24,961)	27,254	10,415
Interest expense and financing costs	(15,677)	(15,800)	(46,982)	(40,185)
Income (loss) before income taxes	225,042	257,950	(35,086)	610,711
Net loss and loss expense ratio	60.30%	55.60%	84.70%	59.10%
Acquisition cost ratio	17.50%	16.30%	17.40%	16.60%
General and administrative expense ratio	13.70%	13.70%	14.20%	14.10%
Combined ratio	91.50%	85.60%	116.30%	89.80%
Goodwill and intangible assets	98,260	89,744	98,260	89,744	103,231
Insurance [Member]
Segment Information [Line Items]
Gross premiums written	493,460	433,550	1,600,548	1,419,372
Net premiums written	331,857	309,277	1,116,222	982,969
Net premiums earned	370,520	320,184	1,058,042	878,117
Other insurance related income	1,156	884	2,047	1,727
Net losses and loss expenses	(207,403)	(150,860)	(692,255)	(437,057)
Acquisition costs	(51,753)	(38,962)	(145,075)	(110,670)
General and administrative expenses	(72,005)	(64,147)	(209,960)	(189,802)
Underwriting income (loss)	40,515	67,099	12,799	142,315
Net loss and loss expense ratio	56.00%	47.10%	65.40%	49.80%
Acquisition cost ratio	14.00%	12.20%	13.70%	12.60%
General and administrative expense ratio	19.40%	20.00%	19.90%	21.60%
Combined ratio	89.40%	79.30%	99.00%	84.00%
Goodwill and intangible assets	98,260	89,744	98,260	89,744
Reinsurance [Member]
Segment Information [Line Items]
Gross premiums written	341,596	317,137	1,829,101	1,696,389
Net premiums written	341,596	317,045	1,808,150	1,675,927
Net premiums earned	469,472	438,689	1,410,165	1,311,975
Other insurance related income	0	0	0	0
Net losses and loss expenses	(299,436)	(271,294)	(1,399,343)	(856,730)
Acquisition costs	(95,083)	(84,826)	(285,022)	(253,944)
General and administrative expenses	(25,439)	(22,292)	(80,900)	(66,960)
Underwriting income (loss)	49,514	60,277	(355,100)	134,341
Net loss and loss expense ratio	63.80%	61.80%	99.20%	65.30%
Acquisition cost ratio	20.30%	19.40%	20.20%	19.40%
General and administrative expense ratio	5.40%	5.10%	5.80%	5.10%
Combined ratio	89.50%	86.30%	125.20%	89.80%
Goodwill and intangible assets	0	0	0	0
Unallocated To Segment [Member]
Segment Information [Line Items]
Corporate expenses	(17,093)	(16,996)	(58,302)	(52,504)
Net investment income	49,396	111,800	260,068	299,004
Net realized investment gains	57,557	76,531	125,177	117,325
Foreign exchange (losses) gains	60,830	(24,961)	27,254	10,415
Interest expense and financing costs	$ (15,677)	$ (15,800)	$ (46,982)	$ (40,185)