SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) reportable_segment	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Information [Line Items]
Number of reportable segments | reportable_segment	2
Net premiums earned	$ 938,703	$ 902,340
Other insurance related income (loss)	(3,783)	(203)
Net losses and loss expenses	(606,942)	(498,962)
Acquisition costs	(189,792)	(180,635)
Expenses	(161,260)	(149,901)
Net investment income	98,664	49,164
Net realized investment losses	(25,050)	(66,508)
Foreign exchange losses	(21,465)	(616)
Interest expense and financing costs	(12,791)	(12,833)
Income before income taxes and interest in income (loss) of equity method investments	$ 16,284	$ 41,846
Net loss and loss expense ratio	64.70%	55.30%
Acquisition cost ratio	20.20%	20.00%
General and administrative expense ratio	17.20%	16.60%
Combined ratio	102.10%	91.90%
Goodwill and intangible assets	$ 84,613	$ 86,446	$ 85,049
Operating Segments [Member]
Segment Information [Line Items]
Gross premiums written	1,911,871	1,959,161
Net premiums written	1,508,959	1,685,806
Net premiums earned	938,703	902,340
Other insurance related income (loss)	(3,783)	(203)
Net losses and loss expenses	(606,942)	(498,962)
Acquisition costs	(189,792)	(180,635)
Expenses	(121,801)	(123,589)
Underwriting income	16,385	98,951
Operating Segments [Member] | Insurance [Member]
Segment Information [Line Items]
Gross premiums written	694,006	653,349
Net premiums written	505,329	473,163
Net premiums earned	458,430	438,678
Other insurance related income (loss)	42	137
Net losses and loss expenses	(286,903)	(274,405)
Acquisition costs	(68,157)	(61,398)
Expenses	(90,448)	(85,576)
Underwriting income	$ 12,964	$ 17,436
Net loss and loss expense ratio	62.60%	62.60%
Acquisition cost ratio	14.90%	14.00%
General and administrative expense ratio	19.70%	19.50%
Combined ratio	97.20%	96.10%
Goodwill and intangible assets	$ 84,613	$ 86,446
Operating Segments [Member] | Reinsurance [Member]
Segment Information [Line Items]
Gross premiums written	1,217,865	1,305,812
Net premiums written	1,003,630	1,212,643
Net premiums earned	480,273	463,662
Other insurance related income (loss)	(3,825)	(340)
Net losses and loss expenses	(320,039)	(224,557)
Acquisition costs	(121,635)	(119,237)
Expenses	(31,353)	(38,013)
Underwriting income	$ 3,421	$ 81,515
Net loss and loss expense ratio	66.60%	48.40%
Acquisition cost ratio	25.30%	25.70%
General and administrative expense ratio	6.60%	8.20%
Combined ratio	98.50%	82.30%
Goodwill and intangible assets	$ 0	$ 0
Corporate, Non-Segment [Member]
Segment Information [Line Items]
Expenses	(39,459)	(26,312)
Significant Reconciling Items [Member]
Segment Information [Line Items]
Net investment income	98,664	49,164
Net realized investment losses	(25,050)	(66,508)
Foreign exchange losses	(21,465)	(616)
Interest expense and financing costs	$ (12,791)	$ (12,833)