SEGMENT INFORMATION (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 reportable_segment	Dec. 31, 2012	Dec. 31, 2011
Segment Information [Line Items]
Number of reportable segments									2
Gross premiums written									$ 4,697,041	$ 4,139,643	$ 4,096,153
Net premiums written									3,928,200	3,337,456	3,419,434
Net premiums earned	941,911	945,242	945,873	874,039	856,049	862,447	850,603	846,362	3,707,065	3,415,463	3,314,961
Other insurance related income									4,424	2,676	2,396
Net losses and loss expenses									(2,134,195)	(2,096,028)	(2,675,052)
Acquisition costs									(664,191)	(627,653)	(587,469)
General and administrative expenses									(575,390)	(560,981)	(459,151)
Underwriting income (loss)	94,159	153,088	9,921	170,799	(74,291)	154,999	119,739	62,689
Net investment income	113,863	103,429	83,112	108,908	86,847	103,638	74,449	116,023	409,312	380,957	362,430
Net realized investment gains	19,558	(4,708)	16,235	44,478	31,771	50,803	30,405	14,491	75,564	127,469	121,439
Foreign exchange (losses) gains									(26,143)	(29,512)	44,582
Interest expense and financing costs									(61,979)	(61,863)	(62,598)
Income before income taxes									734,467	550,528	61,538
Net loss and loss expense ratio									57.60%	61.40%	80.70%
Acquisition cost ratio									17.90%	18.40%	17.70%
General and administrative expense ratio									15.50%	16.40%	13.90%
Combined ratio									91.00%	96.20%	112.30%
Goodwill and intangible assets	89,528				97,493				89,528	97,493	99,590
Insurance [Member]
Segment Information [Line Items]
Net premiums earned									1,722,762	1,558,058	1,429,687
Reinsurance [Member]
Segment Information [Line Items]
Net premiums earned									1,984,303	1,857,405	1,885,274
Operating Segments [Member]
Segment Information [Line Items]
Gross premiums written									4,697,041	4,139,643	4,096,153
Net premiums written									3,928,200	3,337,456	3,419,434
Net premiums earned									3,707,065	3,415,463	3,314,961
Other insurance related income									4,424	2,676	2,396
Net losses and loss expenses									(2,134,195)	(2,096,028)	(2,675,052)
Acquisition costs									(664,191)	(627,653)	(587,469)
General and administrative expenses									(485,134)	(431,321)	(382,062)
Underwriting income (loss)									427,969	263,137	(327,226)
Operating Segments [Member] | Insurance [Member]
Segment Information [Line Items]
Gross premiums written									2,559,138	2,309,481	2,121,829
Net premiums written									1,813,538	1,522,245	1,466,134
Net premiums earned									1,722,762	1,558,058	1,429,687
Other insurance related income									2,436	2,676	2,396
Net losses and loss expenses									(1,050,402)	(953,564)	(919,319)
Acquisition costs									(242,363)	(226,859)	(199,583)
General and administrative expenses									(347,684)	(314,834)	(278,147)
Underwriting income (loss)									84,749	65,477	35,034
Net loss and loss expense ratio									61.00%	61.20%	64.30%
Acquisition cost ratio									14.10%	14.60%	14.00%
General and administrative expense ratio									20.10%	20.20%	19.40%
Combined ratio									95.20%	96.00%	97.70%
Goodwill and intangible assets	89,528				97,493				89,528	97,493	99,590
Operating Segments [Member] | Reinsurance [Member]
Segment Information [Line Items]
Gross premiums written									2,137,903	1,830,162	1,974,324
Net premiums written									2,114,662	1,815,211	1,953,300
Net premiums earned									1,984,303	1,857,405	1,885,274
Other insurance related income									1,988	0	0
Net losses and loss expenses									(1,083,793)	(1,142,464)	(1,755,733)
Acquisition costs									(421,828)	(400,794)	(387,886)
General and administrative expenses									(137,450)	(116,487)	(103,915)
Underwriting income (loss)									343,220	197,660	(362,260)
Net loss and loss expense ratio									54.60%	61.50%	93.10%
Acquisition cost ratio									21.30%	21.60%	20.60%
General and administrative expense ratio									6.90%	6.30%	5.50%
Combined ratio									82.80%	89.40%	119.20%
Goodwill and intangible assets	0				0				0	0	0
Corporate [Member]
Segment Information [Line Items]
General and administrative expenses									(90,256)	(129,660)	(77,089)
Significant Reconciling Items [Member]
Segment Information [Line Items]
Net investment income									409,312	380,957	362,430
Net realized investment gains									75,564	127,469	121,439
Foreign exchange (losses) gains									(26,143)	(29,512)	44,582
Interest expense and financing costs									$ (61,979)	$ (61,863)	$ (62,598)