SEGMENT INFORMATION (DETAILS) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) reportable_segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Information [Line Items]
Number of reportable segments | reportable_segment									2
Gross premiums written									$ 4,970,208	$ 4,603,730	$ 4,711,519
Net premiums written									3,752,974	3,674,666	3,906,975
Net premiums earned	$ 921,879	$ 934,415	$ 946,990	$ 902,340	$ 921,812	$ 919,341	$ 941,211	$ 904,053	3,705,625	3,686,417	3,870,999
Other insurance related income (loss)									7,222	(2,953)	650
Net losses and loss expenses									(2,204,197)	(2,176,199)	(2,186,722)
Acquisition costs									(746,876)	(718,112)	(737,197)
General and administrative expenses									(602,717)	(596,821)	(621,876)
Underwriting income	66,265	103,998	9,860	98,951	88,358	56,245	56,848	100,790
Net investment income	95,517	116,923	91,730	49,164	79,000	45,685	88,544	92,107	353,335	305,336	342,766
Net realized investment losses	(20,229)	$ 5,205	$ 21,010	$ (66,508)	(14,872)	$ (69,957)	$ (11,110)	$ (42,553)	(60,525)	(138,491)	132,108
Foreign exchange gains									121,295	102,312	104,439
Interest expense and financing costs									(51,360)	(50,963)	(74,695)
Termination fee received									0	280,000	0
Reorganization and related expenses									0	(45,867)	0
Income before income taxes and interest in income (loss) of equity method investments									$ 521,802	$ 644,659	$ 830,472
Net loss and loss expense ratio									59.50%	59.00%	56.50%
Acquisition cost ratio									20.20%	19.50%	19.00%
General and administrative expense ratio									16.20%	16.20%	16.10%
Combined ratio									95.90%	94.70%	91.60%
Goodwill and intangible assets	85,049				86,858				$ 85,049	$ 86,858	$ 88,960
Insurance [Member]
Segment Information [Line Items]
Net premiums earned									1,777,321	1,798,191	1,830,544
Reinsurance [Member]
Segment Information [Line Items]
Net premiums earned									1,928,304	1,888,226	2,040,455
Operating Segments [Member]
Segment Information [Line Items]
Gross premiums written									4,970,208	4,603,730	4,711,519
Net premiums written									3,752,974	3,674,666	3,906,975
Net premiums earned									3,705,625	3,686,417	3,870,999
Other insurance related income (loss)									7,222	(2,953)	650
Net losses and loss expenses									(2,204,197)	(2,176,199)	(2,186,722)
Acquisition costs									(746,876)	(718,112)	(737,197)
General and administrative expenses									(482,701)	(486,911)	(486,201)
Underwriting income									279,073	302,242	461,529
Operating Segments [Member] | Insurance [Member]
Segment Information [Line Items]
Gross premiums written									2,720,242	2,583,081	2,535,415
Net premiums written									1,807,125	1,759,359	1,779,501
Net premiums earned									1,777,321	1,798,191	1,830,544
Other insurance related income (loss)									89	1,036	(11)
Net losses and loss expenses									(1,141,933)	(1,154,928)	(1,131,880)
Acquisition costs									(251,120)	(261,208)	(278,804)
General and administrative expenses									(346,857)	(341,658)	(341,214)
Underwriting income									$ 37,500	$ 41,433	$ 78,635
Net loss and loss expense ratio									64.30%	64.20%	61.80%
Acquisition cost ratio									14.10%	14.50%	15.20%
General and administrative expense ratio									19.50%	19.10%	18.70%
Combined ratio									97.90%	97.80%	95.70%
Goodwill and intangible assets	85,049				86,858				$ 85,049	$ 86,858	$ 88,960
Operating Segments [Member] | Reinsurance [Member]
Segment Information [Line Items]
Gross premiums written									2,249,966	2,020,649	2,176,104
Net premiums written									1,945,849	1,915,307	2,127,474
Net premiums earned									1,928,304	1,888,226	2,040,455
Other insurance related income (loss)									7,133	(3,989)	661
Net losses and loss expenses									(1,062,264)	(1,021,271)	(1,054,842)
Acquisition costs									(495,756)	(456,904)	(458,393)
General and administrative expenses									(135,844)	(145,253)	(144,987)
Underwriting income									$ 241,573	$ 260,809	$ 382,894
Net loss and loss expense ratio									55.10%	54.10%	51.70%
Acquisition cost ratio									25.70%	24.20%	22.50%
General and administrative expense ratio									7.00%	7.70%	7.10%
Combined ratio									87.80%	86.00%	81.30%
Goodwill and intangible assets	$ 0				$ 0				$ 0	$ 0	$ 0
Corporate [Member]
Segment Information [Line Items]
General and administrative expenses									(120,016)	(109,910)	(135,675)
Significant Reconciling Items [Member]
Segment Information [Line Items]
Net investment income									353,335	305,336	342,766
Net realized investment losses									(60,525)	(138,491)	132,108
Foreign exchange gains									121,295	102,312	104,439
Interest expense and financing costs									$ (51,360)	(50,963)	$ (74,695)
Termination fee received										280,000
Reorganization and related expenses										$ (45,867)