SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) reportable_segment	Sep. 30, 2018 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | reportable_segment			2
Segment Information [Line Items]
Net premiums earned	$ 1,157,307	$ 1,224,075	$ 3,415,126	$ 3,577,026
Other insurance related income	1,533	8,475	11,385	18,811
Net losses and loss expenses	(850,913)	(794,959)	(2,187,403)	(2,162,945)
Acquisition costs	(260,026)	(248,314)	(762,807)	(709,527)
Expenses	(155,522)	(154,894)	(496,008)	(489,944)
Net investment income	115,763	114,421	361,014	325,380
Net investment gains (losses)	14,527	(17,628)	48,522	(77,551)
Foreign exchange (losses) gains	59,543	(8,305)	64,868	(2,066)
Interest expense and financing costs	(18,042)	(16,897)	(49,545)	(50,758)
Transaction and reorganization expenses	(11,215)	(16,300)	(29,310)	(48,125)
Amortization of value of business acquired	(4,368)	(39,018)	(24,666)	(149,535)
Amortization of intangible assets	(2,831)	(1,753)	(8,744)	(8,564)
Income before income taxes and interest in income of equity method investments	45,756	48,903	342,432	222,202
Operating Segments
Segment Information [Line Items]
Gross premiums written	1,406,506	1,423,707	5,637,491	5,737,327
Net premiums written	856,081	919,938	3,703,460	3,906,264
Net premiums earned	1,157,307	1,224,075	3,415,126	3,577,026
Other insurance related income	1,533	8,475	11,385	18,811
Net losses and loss expenses	(850,913)	(794,959)	(2,187,403)	(2,162,945)
Acquisition costs	(260,026)	(248,314)	(762,807)	(709,527)
Expenses	(126,619)	(130,251)	(398,540)	(404,875)
Underwriting income (loss)	$ (78,718)	$ 59,026	$ 77,761	$ 318,490
Net losses and loss expenses ratio	73.50%	64.90%	64.10%	60.50%
Acquisition cost ratio	22.50%	20.30%	22.30%	19.80%
General and administrative expense ratio	13.40%	12.70%	14.50%	13.70%
Combined ratio	109.40%	97.90%	100.90%	94.00%
Total intangible assets	$ 346,356	$ 408,441	$ 346,356	$ 408,441
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	894,902	969,364	2,714,322	2,876,856
Net premiums written	517,050	602,070	1,638,197	1,748,142
Net premiums earned	536,451	614,795	1,630,473	1,772,126
Other insurance related income	733	1,526	1,779	3,359
Net losses and loss expenses	(338,966)	(415,488)	(961,444)	(1,065,799)
Acquisition costs	(115,551)	(111,888)	(344,981)	(290,082)
Expenses	(100,559)	(100,656)	(311,491)	(305,394)
Underwriting income (loss)	$ (17,892)	$ (11,711)	$ 14,336	$ 114,210
Net losses and loss expenses ratio	63.20%	67.60%	59.00%	60.10%
Acquisition cost ratio	21.50%	18.20%	21.20%	16.40%
General and administrative expense ratio	18.80%	16.40%	19.00%	17.20%
Combined ratio	103.50%	102.20%	99.20%	93.70%
Total intangible assets	$ 346,356	$ 408,441	$ 346,356	$ 408,441
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	511,604	454,343	2,923,169	2,860,471
Net premiums written	339,031	317,868	2,065,263	2,158,122
Net premiums earned	620,856	609,280	1,784,653	1,804,900
Other insurance related income	800	6,949	9,606	15,452
Net losses and loss expenses	(511,947)	(379,471)	(1,225,959)	(1,097,146)
Acquisition costs	(144,475)	(136,426)	(417,826)	(419,445)
Expenses	(26,060)	(29,595)	(87,049)	(99,481)
Underwriting income (loss)	$ (60,826)	$ 70,737	$ 63,425	$ 204,280
Net losses and loss expenses ratio	82.50%	62.30%	68.70%	60.80%
Acquisition cost ratio	23.30%	22.40%	23.40%	23.20%
General and administrative expense ratio	4.10%	4.80%	4.90%	5.50%
Combined ratio	109.90%	89.50%	97.00%	89.50%
Total intangible assets	$ 0	$ 0	$ 0	$ 0
Corporate Expenses
Segment Information [Line Items]
Expenses	(28,903)	(24,643)	(97,468)	(85,069)
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	115,763	114,421	361,014	325,380
Net investment gains (losses)	14,527	(17,628)	48,522	(77,551)
Foreign exchange (losses) gains	59,543	(8,305)	64,868	(2,066)
Interest expense and financing costs	(18,042)	(16,897)	(49,545)	(50,758)
Transaction and reorganization expenses	(11,215)	(16,300)	(29,310)	(48,125)
Amortization of value of business acquired	(4,368)	(39,018)	(24,666)	(149,535)
Amortization of intangible assets	$ (2,831)	$ (1,753)	$ (8,744)	$ (8,564)