INVESTMENTS - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
INVESTMENTS
Accrued interest receivable	$ 14,944	$ 14,587
Available-for-sale
Total amortized cost	1,912,851	1,915,278
Allowance for credit losses	(878)	0
Available-for-sale Securities, Debt Securities	1,963,585	1,983,086
Amortized Cost
Due in one year or less	56,844
Due after one year through five years	442,572
Due after five years through 10 years	549,193
Due after 10 years	236,938
Mtge/ABS/CMBS	627,304
Total amortized cost	1,912,851	1,915,278
Fair Value
Due in one year or less	56,702
Due after one year through five years	449,447
Due after five years through 10 years	569,898
Due after 10 years	249,675
Mtge/ABS/CMBS	637,863
Total fair value	1,963,585	1,983,086
Fair value measured on recurring basis
INVESTMENTS
Total assets at fair value	2,332,044	2,443,716
Available-for-sale
Available-for-sale Securities, Debt Securities	1,963,585	1,983,086
Fair Value
Total fair value	1,963,585	1,983,086
Fair value measured on recurring basis | Significant Unobservable Inputs (Level 3)
INVESTMENTS
Total assets at fair value	4,803	1,770
Available-for-sale
Available-for-sale Securities, Debt Securities	4,803	1,770
Fair Value
Total fair value	4,803	1,770
Debt securities
INVESTMENTS
Accrued interest receivable	13,900	13,500
Available-for-sale
Total amortized cost	1,912,851	1,915,278
Allowance for credit losses	(878)
Gross unrealized gains	80,141	70,461
Gross unrealized losses	(28,529)	(2,653)
Available-for-sale Securities, Debt Securities	1,963,585	1,983,086
Amortized Cost
Total amortized cost	1,912,851	1,915,278
Fair Value
Total fair value	1,963,585	1,983,086
U.S. government
Available-for-sale
Total amortized cost	171,026	186,699
Gross unrealized gains	15,948	6,994
Gross unrealized losses		(32)
Available-for-sale Securities, Debt Securities	186,974	193,661
Amortized Cost
Total amortized cost	171,026	186,699
Fair Value
Total fair value	186,974	193,661
U.S. Agency
Available-for-sale
Total amortized cost	32,236	36,535
Gross unrealized gains	4,806	2,362
Gross unrealized losses		(42)
Available-for-sale Securities, Debt Securities	37,042	38,855
Amortized Cost
Total amortized cost	32,236	36,535
Fair Value
Total fair value	37,042	38,855
Non-U.S. govt. & agency
Available-for-sale
Total amortized cost	7,324	7,333
Gross unrealized gains	105	295
Gross unrealized losses	(113)
Available-for-sale Securities, Debt Securities	7,316	7,628
Amortized Cost
Total amortized cost	7,324	7,333
Fair Value
Total fair value	7,316	7,628
Agency MBS
Available-for-sale
Total amortized cost	396,474	411,808
Gross unrealized gains	17,807	8,920
Gross unrealized losses	(19)	(563)
Available-for-sale Securities, Debt Securities	414,262	420,165
Amortized Cost
Total amortized cost	396,474	411,808
Fair Value
Total fair value	414,262	420,165
ABS/CMBS
Available-for-sale
Total amortized cost	230,830	222,832
Allowance for credit losses	(21)
Gross unrealized gains	1,908	2,514
Gross unrealized losses	(9,116)	(476)
Available-for-sale Securities, Debt Securities	223,601	224,870
Amortized Cost
Total amortized cost	230,830	222,832
Fair Value
Total fair value	223,601	224,870
Corporate Debt
Available-for-sale
Total amortized cost	685,116	659,640
Allowance for credit losses	(857)
Gross unrealized gains	23,364	33,245
Gross unrealized losses	(19,205)	(818)
Available-for-sale Securities, Debt Securities	688,418	692,067
Amortized Cost
Total amortized cost	685,116	659,640
Fair Value
Total fair value	688,418	692,067
Municipal
Available-for-sale
Total amortized cost	389,845	390,431
Gross unrealized gains	16,203	16,131
Gross unrealized losses	(76)	(722)
Available-for-sale Securities, Debt Securities	405,972	405,840
Amortized Cost
Total amortized cost	389,845	390,431
Fair Value
Total fair value	$ 405,972	$ 405,840