INVESTMENTS - Gross unrealized losses (Details) $ in Thousands	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Fixed maturities
Total Unrealized Losses	$ (215,000)	$ (91,000)
Fixed maturities
Fixed maturities
Fair Value - 12 months or greater	3,569,519	1,532,513
Unrealized Losses - 12 months or greater	(114,561)	(44,352)
Fair Value - Less than 12 months	4,917,951	5,619,002
Unrealized Losses - Less than 12 months	(100,278)	(46,521)
Total Fair Value	8,487,470	7,151,515
Total Unrealized Losses	(214,839)	(90,873)
Equity securities
Fair Value - 12 months or greater	$ 3,570,000	$ 1,533,000
Unrealized loss position, number of positions | security	3,599	2,424
Continuous unrealized loss position, 12 months or longer, number of positions | security	1,656	627
Fixed maturities | Below Investment Grade or Not Rated
Fixed maturities
Total Unrealized Losses	$ (49,000)	$ (7,000)
U.S. government and agency
Fixed maturities
Fair Value - 12 months or greater	374,030	194,916
Unrealized Losses - 12 months or greater	(7,659)	(5,963)
Fair Value - Less than 12 months	424,439	1,389,792
Unrealized Losses - Less than 12 months	(1,018)	(10,946)
Total Fair Value	798,469	1,584,708
Total Unrealized Losses	(8,677)	(16,909)
Non-U.S. government
Fixed maturities
Fair Value - 12 months or greater	44,339	62,878
Unrealized Losses - 12 months or greater	(2,004)	(6,806)
Fair Value - Less than 12 months	303,376	204,110
Unrealized Losses - Less than 12 months	(14,116)	(2,717)
Total Fair Value	347,715	266,988
Total Unrealized Losses	(16,120)	(9,523)
Corporate debt
Fixed maturities
Fair Value - 12 months or greater	1,439,378	407,300
Unrealized Losses - 12 months or greater	(58,915)	(11,800)
Fair Value - Less than 12 months	2,547,135	2,041,845
Unrealized Losses - Less than 12 months	(69,529)	(18,051)
Total Fair Value	3,986,513	2,449,145
Total Unrealized Losses	(128,444)	(29,851)
Agency RMBS
Fixed maturities
Fair Value - 12 months or greater	940,645	759,255
Unrealized Losses - 12 months or greater	(29,255)	(17,453)
Fair Value - Less than 12 months	117,181	1,172,313
Unrealized Losses - Less than 12 months	(629)	(10,247)
Total Fair Value	1,057,826	1,931,568
Total Unrealized Losses	(29,884)	(27,700)
CMBS
Fixed maturities
Fair Value - 12 months or greater	455,582	31,607
Unrealized Losses - 12 months or greater	(11,430)	(703)
Fair Value - Less than 12 months	353,802	348,943
Unrealized Losses - Less than 12 months	(2,365)	(2,422)
Total Fair Value	809,384	380,550
Total Unrealized Losses	(13,795)	(3,125)
Non-Agency RMBS
Fixed maturities
Fair Value - 12 months or greater	9,494	8,029
Unrealized Losses - 12 months or greater	(1,170)	(788)
Fair Value - Less than 12 months	11,432	4,197
Unrealized Losses - Less than 12 months	(112)	(11)
Total Fair Value	20,926	12,226
Total Unrealized Losses	(1,282)	(799)
ABS
Fixed maturities
Fair Value - 12 months or greater	237,237	57,298
Unrealized Losses - 12 months or greater	(2,755)	(570)
Fair Value - Less than 12 months	1,150,692	392,170
Unrealized Losses - Less than 12 months	(12,485)	(1,424)
Total Fair Value	1,387,929	449,468
Total Unrealized Losses	(15,240)	(1,994)
Municipals
Fixed maturities
Fair Value - 12 months or greater	68,814	11,230
Unrealized Losses - 12 months or greater	(1,373)	(269)
Fair Value - Less than 12 months	9,894	65,632
Unrealized Losses - Less than 12 months	(24)	(703)
Total Fair Value	78,708	76,862
Total Unrealized Losses	$ (1,397)	(972)
Equity securities
Equity securities
Fair Value - 12 months or greater		0
Unrealized Losses - 12 months or greater		0
Fair Value - Less than 12 months		27,709
Unrealized Losses - less than 12 months		(1,883)
Total Fair Value		27,709
Total Unrealized Losses		(1,883)
Common stocks
Equity securities
Fair Value - 12 months or greater		0
Unrealized Losses - 12 months or greater		0
Fair Value - Less than 12 months		3,202
Unrealized Losses - less than 12 months		(590)
Total Fair Value		3,202
Total Unrealized Losses		(590)
Exchange-traded funds
Equity securities
Fair Value - 12 months or greater		0
Unrealized Losses - 12 months or greater		0
Fair Value - Less than 12 months		12,323
Unrealized Losses - less than 12 months		(294)
Total Fair Value		12,323
Total Unrealized Losses		(294)
Bond mutual funds
Equity securities
Fair Value - 12 months or greater		0
Unrealized Losses - 12 months or greater		0
Fair Value - Less than 12 months		12,184
Unrealized Losses - less than 12 months		(999)
Total Fair Value		12,184
Total Unrealized Losses		$ (999)