INVESTMENTS - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
INVESTMENTS
Accrued interest receivable	$ 15,142	$ 14,587
Available-for-sale
Total amortized cost	1,956,137	1,915,278
Allowance for credit losses	(985)	0
Available-for-sale Securities, Debt Securities	2,075,093	1,983,086
Amortized Cost
Due in one year or less	75,077
Due after one year through five years	472,458
Due after five years through 10 years	526,480
Due after 10 years	279,538
Mtge/ABS/CMBS	602,584
Total amortized cost	1,956,137	1,915,278
Fair Value
Due in one year or less	75,888
Due after one year through five years	495,968
Due after five years through 10 years	571,103
Due after 10 years	304,460
Mtge/ABS/CMBS	627,674
Total fair value	2,075,093	1,983,086
Fair value measured on recurring basis
INVESTMENTS
Total assets at fair value	2,513,083	2,443,716
Available-for-sale
Available-for-sale Securities, Debt Securities	2,075,093	1,983,086
Fair Value
Total fair value	2,075,093	1,983,086
Fair value measured on recurring basis | Significant Unobservable Inputs (Level 3)
INVESTMENTS
Total assets at fair value	7,059	1,770
Available-for-sale
Available-for-sale Securities, Debt Securities	7,059	1,770
Fair Value
Total fair value	7,059	1,770
Debt securities
INVESTMENTS
Accrued interest receivable	14,100	13,500
Available-for-sale
Total amortized cost	1,956,137	1,915,278
Allowance for credit losses	(985)
Gross unrealized gains	127,620	70,461
Gross unrealized losses	(7,679)	(2,653)
Available-for-sale Securities, Debt Securities	2,075,093	1,983,086
Amortized Cost
Total amortized cost	1,956,137	1,915,278
Fair Value
Total fair value	2,075,093	1,983,086
U.S. government
Available-for-sale
Total amortized cost	171,174	186,699
Gross unrealized gains	15,664	6,994
Gross unrealized losses		(32)
Available-for-sale Securities, Debt Securities	186,838	193,661
Amortized Cost
Total amortized cost	171,174	186,699
Fair Value
Total fair value	186,838	193,661
U.S. Agency
Available-for-sale
Total amortized cost	27,913	36,535
Gross unrealized gains	4,178	2,362
Gross unrealized losses		(42)
Available-for-sale Securities, Debt Securities	32,091	38,855
Amortized Cost
Total amortized cost	27,913	36,535
Fair Value
Total fair value	32,091	38,855
Non-U.S. govt. & agency
Available-for-sale
Total amortized cost	7,316	7,333
Gross unrealized gains	401	295
Gross unrealized losses	(25)
Available-for-sale Securities, Debt Securities	7,692	7,628
Amortized Cost
Total amortized cost	7,316	7,333
Fair Value
Total fair value	7,692	7,628
Agency MBS
Available-for-sale
Total amortized cost	388,036	411,808
Gross unrealized gains	21,208	8,920
Gross unrealized losses	(77)	(563)
Available-for-sale Securities, Debt Securities	409,167	420,165
Amortized Cost
Total amortized cost	388,036	411,808
Fair Value
Total fair value	409,167	420,165
ABS/CMBS
Available-for-sale
Total amortized cost	214,548	222,832
Allowance for credit losses	(54)
Gross unrealized gains	5,218	2,514
Gross unrealized losses	(1,205)	(476)
Available-for-sale Securities, Debt Securities	218,507	224,870
Amortized Cost
Total amortized cost	214,548	222,832
Fair Value
Total fair value	218,507	224,870
Corporate Debt
Available-for-sale
Total amortized cost	717,748	659,640
Allowance for credit losses	(931)
Gross unrealized gains	54,610	33,245
Gross unrealized losses	(6,302)	(818)
Available-for-sale Securities, Debt Securities	765,125	692,067
Amortized Cost
Total amortized cost	717,748	659,640
Fair Value
Total fair value	765,125	692,067
Municipal
Available-for-sale
Total amortized cost	429,402	390,431
Gross unrealized gains	26,341	16,131
Gross unrealized losses	(70)	(722)
Available-for-sale Securities, Debt Securities	455,673	405,840
Amortized Cost
Total amortized cost	429,402	390,431
Fair Value
Total fair value	$ 455,673	$ 405,840