SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) reportable_segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | reportable_segment	2
Segment Information [Line Items]
Net premiums earned	$ 1,088,625	$ 1,134,212
Other insurance related income (loss)	(8,707)	6,929
Net losses and loss expenses	(908,073)	(664,028)
Acquisition costs	(238,650)	(260,418)
Expenses	(157,060)	(175,091)
Net investment income	93,101	107,303
Net investment gains (losses)	(62,877)	12,767
Foreign exchange (losses) gains	61,683	(7,056)
Interest expense and financing costs	(23,472)	(15,895)
Reorganization expenses	982	(14,820)	$ (15,000)
Amortization of value of business acquired	(1,799)	(13,104)
Amortization of intangible assets	(2,870)	(3,003)
Income before income taxes and interest in income of equity method investments	(159,117)	107,796
Operating Segments
Segment Information [Line Items]
Gross premiums written	2,431,158	2,583,226
Net premiums written	1,679,044	1,777,059
Net premiums earned	1,088,625	1,134,212
Other insurance related income (loss)	(8,707)	6,929
Net losses and loss expenses	(908,073)	(664,028)
Acquisition costs	(238,650)	(260,418)
Expenses	(129,962)	(138,873)
Underwriting income (loss)	$ (196,767)	$ 77,822
Net losses and loss expenses ratio	83.40%	58.50%
Acquisition cost ratio	21.90%	23.00%
General and administrative expense ratio	14.50%	15.40%
Combined ratio	119.80%	96.90%
Goodwill and intangible assets	$ 329,824	$ 340,766
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	940,715	851,096
Net premiums written	581,650	529,239
Net premiums earned	562,064	556,762
Other insurance related income (loss)	647	1,742
Net losses and loss expenses	(471,812)	(313,776)
Acquisition costs	(112,751)	(117,775)
Expenses	(100,778)	(106,034)
Underwriting income (loss)	$ (122,630)	$ 20,919
Net losses and loss expenses ratio	83.90%	56.40%
Acquisition cost ratio	20.10%	21.20%
General and administrative expense ratio	17.90%	19.00%
Combined ratio	121.90%	96.60%
Goodwill and intangible assets	$ 329,824	$ 340,766
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	1,490,443	1,732,130
Net premiums written	1,097,394	1,247,820
Net premiums earned	526,561	577,450
Other insurance related income (loss)	(9,354)	5,187
Net losses and loss expenses	(436,261)	(350,252)
Acquisition costs	(125,899)	(142,643)
Expenses	(29,184)	(32,839)
Underwriting income (loss)	$ (74,137)	$ 56,903
Net losses and loss expenses ratio	82.90%	60.70%
Acquisition cost ratio	23.90%	24.70%
General and administrative expense ratio	5.50%	5.60%
Combined ratio	112.30%	91.00%
Goodwill and intangible assets	$ 0	$ 0
Corporate Expenses
Segment Information [Line Items]
Expenses	(27,098)	(36,218)
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	93,101	107,303
Net investment gains (losses)	(62,877)	12,767
Foreign exchange (losses) gains	61,683	(7,056)
Interest expense and financing costs	(23,472)	(15,895)
Reorganization expenses	982	(14,820)
Amortization of value of business acquired	(1,799)	(13,104)
Amortization of intangible assets	$ (2,870)	$ (3,003)