SEGMENT INFORMATION (DETAILS) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Information [Line Items]
Number of reportable segments	2								2
Gross premiums written									$ 4,096,153	$ 3,750,536	$ 3,587,295
Net premiums written									3,419,434	3,147,540	2,816,429
Net premiums earned	846,753	839,992	840,014	788,201	757,318	758,873	735,027	696,192	3,314,961	2,947,410	2,791,764
Other insurance related income (loss)									2,396	2,073	(129,681)
Net losses and loss expenses									(2,675,052)	(1,677,132)	(1,423,872)
Acquisition costs									(587,469)	(488,712)	(420,495)
General and administrative expenses									(382,062)	(374,436)	(293,081)
Underwriting income (loss)	15,076	90,029	28,974	(461,305)	132,546	127,376	120,836	28,446	(327,226)	409,203	524,635
Corporate expenses									(77,089)	(75,449)	(77,076)
Net investment income	102,362	49,396	100,018	110,655	107,889	111,800	82,584	104,619	362,430	406,892	464,478
Net realized investment gains (losses)	(3,738)	57,557	37,477	30,144	77,772	76,531	24,619	16,176	121,439	195,098	(311,584)
Foreign exchange (losses) gains									44,582	15,535	(28,561)
Interest expense and financing costs									(62,598)	(55,876)	(32,031)
Income before income taxes									61,538	895,403	539,861
Net loss and loss expense ratio									80.70%	56.90%	51.00%
Acquisition cost ratio									17.70%	16.60%	15.10%
General and administrative expense ratio									13.90%	15.20%	13.20%
Combined ratio									112.30%	88.70%	79.30%
Goodwill and intangible assets	99,590				103,231				99,590	103,231	91,505
Insurance [Member]
Segment Information [Line Items]
Gross premiums written									2,121,829	1,916,116	1,775,590
Net premiums written									1,466,134	1,332,220	1,025,061
Net premiums earned									1,429,687	1,206,493	1,157,966
Other insurance related income (loss)									2,396	2,073	(130,946)
Net losses and loss expenses									(919,319)	(569,869)	(612,694)
Acquisition costs									(199,583)	(152,223)	(113,187)
General and administrative expenses									(278,147)	(276,435)	(216,954)
Underwriting income (loss)									35,034	210,039	84,185
Net loss and loss expense ratio									64.30%	47.20%	52.90%
Acquisition cost ratio									14.00%	12.60%	9.80%
General and administrative expense ratio									19.40%	23.00%	18.70%
Combined ratio									97.70%	82.80%	81.40%
Goodwill and intangible assets	99,590				103,231				99,590	103,231	91,505
Reinsurance [Member]
Segment Information [Line Items]
Gross premiums written									1,974,324	1,834,420	1,811,705
Net premiums written									1,953,300	1,815,320	1,791,368
Net premiums earned									1,885,274	1,740,917	1,633,798
Other insurance related income (loss)									0	0	1,265
Net losses and loss expenses									(1,755,733)	(1,107,263)	(811,178)
Acquisition costs									(387,886)	(336,489)	(307,308)
General and administrative expenses									(103,915)	(98,001)	(76,127)
Underwriting income (loss)									(362,260)	199,164	440,450
Net loss and loss expense ratio									93.10%	63.60%	49.60%
Acquisition cost ratio									20.60%	19.30%	18.80%
General and administrative expense ratio									5.50%	5.70%	4.70%
Combined ratio									119.20%	88.60%	73.10%
Goodwill and intangible assets	0				0				0	0	0
Unallocated To Segment [Member]
Segment Information [Line Items]
Corporate expenses									(77,089)	(75,449)	(77,076)
Net investment income									362,430	406,892	464,478
Net realized investment gains (losses)									121,439	195,098	(311,584)
Foreign exchange (losses) gains									44,582	15,535	(28,561)
Interest expense and financing costs									$ (62,598)	$ (55,876)	$ (32,031)