SEGMENT INFORMATION (DETAILS) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 reportable_segment	Jun. 30, 2013	Dec. 31, 2013
Segment Information [Line Items]
Number of reportable segments			2
Net premiums earned	$ 1,000,400	$ 945,873	$ 1,946,349	$ 1,819,911
Other insurance related income	1,683	435	4,766	1,030
Net losses and loss expenses	(565,829)	(642,899)	(1,110,036)	(1,081,313)
Acquisition costs	(191,862)	(169,719)	(363,899)	(315,209)
General and administrative expenses	(151,081)	(149,034)	(303,810)	(290,508)
Net investment income	114,867	83,112	197,610	192,019
Net realized investment gains	33,261	16,235	43,882	60,713
Foreign exchange (losses) gains	(9,705)	10,320	(13,939)	45,201
Interest expense and financing costs	(19,975)	(15,260)	(36,569)	(31,095)
Income before income taxes	211,759	79,063	364,354	400,749
Net loss and loss expense ratio	56.60%	68.00%	57.00%	59.40%
Acquisition cost ratio	19.20%	17.90%	18.70%	17.30%
General and administrative expense ratio	15.00%	15.80%	15.60%	16.00%
Combined ratio	90.80%	101.70%	91.30%	92.70%
Goodwill and intangible assets	90,025	91,370	90,025	91,370	89,528
Operating Segments [Member]
Segment Information [Line Items]
Gross premiums written	1,231,279	1,219,805	3,052,678	2,966,287
Net premiums written	1,000,162	993,407	2,664,746	2,563,846
Net premiums earned	1,000,400	945,873	1,946,349	1,819,911
Other insurance related income	1,683	435	4,766	1,030
Net losses and loss expenses	(565,829)	(642,899)	(1,110,036)	(1,081,313)
Acquisition costs	(191,862)	(169,719)	(363,899)	(315,209)
General and administrative expenses	(117,811)	(123,769)	(241,834)	(243,698)
Underwriting income (loss)	126,581	9,921	235,346	180,721
Operating Segments [Member] | Insurance [Member]
Segment Information [Line Items]
Gross premiums written	754,110	781,055	1,355,831	1,377,769
Net premiums written	541,097	559,584	997,789	992,264
Net premiums earned	457,670	422,345	906,884	824,224
Other insurance related income	0	435	0	1,030
Net losses and loss expenses	(290,466)	(330,992)	(569,889)	(548,328)
Acquisition costs	(71,039)	(58,749)	(136,096)	(116,009)
General and administrative expenses	(83,512)	(88,526)	(171,459)	(175,415)
Underwriting income (loss)	12,653	(55,487)	29,440	(14,498)
Net loss and loss expense ratio	63.50%	78.40%	62.80%	66.50%
Acquisition cost ratio	15.50%	13.90%	15.00%	14.10%
General and administrative expense ratio	18.20%	20.90%	19.00%	21.30%
Combined ratio	97.20%	113.20%	96.80%	101.90%
Goodwill and intangible assets	90,025	91,370	90,025	91,370
Operating Segments [Member] | Reinsurance [Member]
Segment Information [Line Items]
Gross premiums written	477,169	438,750	1,696,847	1,588,518
Net premiums written	459,065	433,823	1,666,957	1,571,582
Net premiums earned	542,730	523,528	1,039,465	995,687
Other insurance related income	1,683	0	4,766	0
Net losses and loss expenses	(275,363)	(311,907)	(540,147)	(532,985)
Acquisition costs	(120,823)	(110,970)	(227,803)	(199,200)
General and administrative expenses	(34,299)	(35,243)	(70,375)	(68,283)
Underwriting income (loss)	113,928	65,408	205,906	195,219
Net loss and loss expense ratio	50.70%	59.60%	52.00%	53.50%
Acquisition cost ratio	22.30%	21.20%	21.90%	20.00%
General and administrative expense ratio	6.30%	6.70%	6.70%	6.90%
Combined ratio	79.30%	87.50%	80.60%	80.40%
Goodwill and intangible assets	0	0	0	0
Corporate, Non-Segment [Member]
Segment Information [Line Items]
General and administrative expenses	(33,270)	(25,265)	(61,976)	(46,810)
Significant Reconciling Items [Member]
Segment Information [Line Items]
Net investment income	114,867	83,112	197,610	192,019
Net realized investment gains	33,261	16,235	43,882	60,713
Foreign exchange (losses) gains	(9,705)	10,320	(13,939)	45,201
Interest expense and financing costs	$ (19,975)	$ (15,260)	$ (36,569)	$ (31,095)