SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) reportable_segment	Jun. 30, 2018 USD ($)
Segment Information [Line Items]
Number of reportable segments | reportable_segment			2
Net premiums earned	$ 1,123,607	$ 1,185,548	$ 2,257,819	$ 2,352,950
Other insurance related income (losses)	2,925	3,730	9,852	10,335
Net losses and loss expenses	(672,463)	(706,641)	(1,336,491)	(1,367,986)
Acquisition costs	(242,363)	(231,952)	(502,781)	(461,212)
Expenses	(165,395)	(165,213)	(340,486)	(335,049)
Net investment income	137,949	109,960	245,254	210,961
Net investment gains (losses)	21,225	(45,093)	33,996	(59,923)
Operating lease expense	4,061	8,000	11,095	14,000
Foreign exchange gains	12,381	44,099	5,325	6,239
Interest expense and financing costs	(15,607)	(17,098)	(31,502)	(33,861)
Transaction and reorganization expenses	(3,276)	(18,772)	(18,096)	(31,825)
Amortization of value of business acquired	(7,194)	(53,407)	(20,298)	(110,517)
Amortization of intangible assets	(2,912)	(4,029)	(5,914)	(6,811)
Income before income taxes and interest in income of equity method investments	$ 188,877	$ 101,132	$ 296,678	$ 173,301
Net losses and loss expenses ratio	59.80%	59.60%	59.20%	58.10%
Acquisition cost ratio	21.60%	19.60%	22.30%	19.60%
General and administrative expense ratio	14.70%	13.90%	15.00%	14.30%
Combined ratio	96.10%	93.10%	96.50%	92.00%
Total intangible assets	$ 353,428	$ 450,073	$ 353,428	$ 450,073
Operating Segments
Segment Information [Line Items]
Gross premiums written	1,647,760	1,650,825	4,230,986	4,313,620
Net premiums written	1,070,321	1,000,455	2,847,381	2,986,326
Net premiums earned	1,123,607	1,185,548	2,257,819	2,352,950
Other insurance related income (losses)	2,925	3,730	9,852	10,335
Net losses and loss expenses	(672,463)	(706,641)	(1,336,491)	(1,367,986)
Acquisition costs	(242,363)	(231,952)	(502,781)	(461,212)
Expenses	(133,047)	(134,959)	(271,920)	(274,624)
Underwriting income	78,659	115,726	156,479	259,463
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	968,325	1,026,644	1,819,421	1,907,492
Net premiums written	591,909	598,179	1,121,149	1,146,071
Net premiums earned	537,260	577,271	1,094,022	1,157,330
Other insurance related income (losses)	(695)	1,214	1,046	1,833
Net losses and loss expenses	(308,703)	(328,773)	(622,479)	(650,312)
Acquisition costs	(111,655)	(90,864)	(229,430)	(178,193)
Expenses	(104,898)	(102,369)	(210,932)	(204,738)
Underwriting income	$ 11,309	$ 56,479	$ 32,227	$ 125,920
Net losses and loss expenses ratio	57.50%	57.00%	56.90%	56.20%
Acquisition cost ratio	20.80%	15.70%	21.00%	15.40%
General and administrative expense ratio	19.50%	17.70%	19.20%	17.70%
Combined ratio	97.80%	90.40%	97.10%	89.30%
Total intangible assets	$ 353,428	$ 450,073	$ 353,428	$ 450,073
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	679,435	624,181	2,411,565	2,406,128
Net premiums written	478,412	402,276	1,726,232	1,840,255
Net premiums earned	586,347	608,277	1,163,797	1,195,620
Other insurance related income (losses)	3,620	2,516	8,806	8,502
Net losses and loss expenses	(363,760)	(377,868)	(714,012)	(717,674)
Acquisition costs	(130,708)	(141,088)	(273,351)	(283,019)
Expenses	(28,149)	(32,590)	(60,988)	(69,886)
Underwriting income	$ 67,350	$ 59,247	$ 124,252	$ 133,543
Net losses and loss expenses ratio	62.00%	62.10%	61.40%	60.00%
Acquisition cost ratio	22.30%	23.20%	23.50%	23.70%
General and administrative expense ratio	4.80%	5.40%	5.20%	5.80%
Combined ratio	89.10%	90.70%	90.10%	89.50%
Total intangible assets	$ 0	$ 0	$ 0	$ 0
Corporate, Non-Segment
Segment Information [Line Items]
Expenses	(32,348)	(30,254)	(68,566)	(60,425)
Significant Reconciling Items
Segment Information [Line Items]
Net investment income	137,949	109,960	245,254	210,961
Net investment gains (losses)	21,225	(45,093)	33,996	(59,923)
Foreign exchange gains	12,381	44,099	5,325	6,239
Interest expense and financing costs	(15,607)	(17,098)	(31,502)	(33,861)
Transaction and reorganization expenses	(3,276)	(18,772)	(18,096)	(31,825)
Amortization of value of business acquired	(7,194)	(53,407)	(20,298)	(110,517)
Amortization of intangible assets	$ (2,912)	$ (4,029)	$ (5,914)	$ (6,811)