UNAUDITED CONDENSED QUARTERLY FINANCIAL DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 1,214,469	$ 1,224,075	$ 1,185,548	$ 1,167,402	$ 1,211,495	$ 1,017,131	$ 981,431	$ 938,703	$ 4,791,495	$ 4,148,760	$ 3,705,625
Net investment income	113,128	114,421	109,960	100,999	100,908	95,169	106,063	98,664	438,507	400,805	353,335
Net investment losses	(72,667)	(17,628)	(45,093)	(14,830)	43,038	14,632	(4,392)	(25,050)	(150,218)	28,226	(60,525)
Underwriting income (loss)	(194,664)	59,026	115,726	143,737	26,130	(512,853)	57,012	16,385
Net income (loss) available (attributable) to common shareholders	$ (198,448)	$ 43,439	$ 92,858	$ 62,546	$ (38,081)	$ (467,740)	$ 85,030	$ 5,014	$ 396	$ (415,779)	$ 465,462
Basic earnings (loss) per common share (in dollars per share)	$ (2.37)	$ 0.52	$ 1.11	$ 0.75	$ (0.46)	$ (5.61)	$ 1.01	$ 0.06	$ 0.00	$ (4.94)	$ 5.13
Diluted earnings (loss) per common share (in dollars per share)	$ (2.37)	$ 0.52	$ 1.11	$ 0.75	$ (0.46)	$ (5.61)	$ 1.01	$ 0.06	$ 0.00	$ (4.94)	$ 5.08
Transaction and reorganization expenses	$ 19,000	$ 16,000	$ 19,000	$ 13,000	$ 21,000	$ 6,000
Amortization of value of business acquired	$ 23,000	$ 39,000	$ 53,000	$ 57,000	50,000				$ 172,332	$ 50,104	$ 0
Total income tax expense (benefit)					$ 42,000				$ (29,486)	$ (7,542)	$ 6,340