INVESTMENTS - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Available-for-sale
Total amortized cost		$ 1,809,429	$ 1,776,465
Available-for-sale Securities, Debt Securities		1,865,613	1,760,515
Amortized Cost
Due in one year or less		51,149
Due after one year through five years		400,628
Due after five years through 10 years		615,648
Due after 10 years		189,428
Mtge/ABS/CMBS	[1]	552,576
Total amortized cost		1,809,429	1,776,465
Fair Value
Due in one year or less		51,233
Due after one year through five years		410,544
Due after five years through 10 years		643,270
Due after 10 years		200,494
Mtge/ABS/CMBS	[1]	560,072
Total fair value		1,865,613	1,760,515
Fair value measured on recurring basis
Available-for-sale
Available-for-sale Securities, Debt Securities		1,865,613	1,760,515
Fair Value
Total fair value		1,865,613	1,760,515
Fair value measured on recurring basis | Significant Unobservable Inputs (Level 3)
INVESTMENTS
Total assets at fair value		0
Debt securities
Available-for-sale
Total amortized cost		1,809,429	1,776,465
Gross unrealized gains		59,818	13,539
Gross unrealized losses		(3,634)	(29,489)
Available-for-sale Securities, Debt Securities		1,865,613	1,760,515
Amortized Cost
Total amortized cost		1,809,429	1,776,465
Fair Value
Total fair value		1,865,613	1,760,515
U.S. government
Available-for-sale
Total amortized cost		194,712	199,982
Gross unrealized gains		6,774	1,232
Gross unrealized losses		(101)	(985)
Available-for-sale Securities, Debt Securities		201,385	200,229
Amortized Cost
Total amortized cost		194,712	199,982
Fair Value
Total fair value		201,385	200,229
U.S. Agency
Available-for-sale
Total amortized cost		29,708	31,716
Gross unrealized gains		1,987	403
Gross unrealized losses			(215)
Available-for-sale Securities, Debt Securities		31,695	31,904
Amortized Cost
Total amortized cost		29,708	31,716
Fair Value
Total fair value		31,695	31,904
Non-U.S. govt. & agency
Available-for-sale
Total amortized cost		9,249	8,170
Gross unrealized gains		192
Gross unrealized losses		(329)	(531)
Available-for-sale Securities, Debt Securities		9,112	7,639
Amortized Cost
Total amortized cost		9,249	8,170
Fair Value
Total fair value		9,112	7,639
Mortgage-backed
Available-for-sale
Total amortized cost		402,993	402,992
Gross unrealized gains		6,603	1,709
Gross unrealized losses		(1,400)	(9,448)
Available-for-sale Securities, Debt Securities		408,196	395,253
Amortized Cost
Total amortized cost		402,993	402,992
Fair Value
Total fair value		408,196	395,253
ABS/CMBS
Available-for-sale
Total amortized cost	[2]	149,583	137,224
Gross unrealized gains	[2]	2,440	375
Gross unrealized losses	[2]	(147)	(876)
Available-for-sale Securities, Debt Securities	[2]	151,876	136,723
Amortized Cost
Total amortized cost	[2]	149,583	137,224
Fair Value
Total fair value	[2]	151,876	136,723
Corporate Debt
Available-for-sale
Total amortized cost		701,358	681,909
Gross unrealized gains		27,072	2,894
Gross unrealized losses		(1,632)	(16,124)
Available-for-sale Securities, Debt Securities		726,798	668,679
Amortized Cost
Total amortized cost		701,358	681,909
Fair Value
Total fair value		726,798	668,679
Municipal
Available-for-sale
Total amortized cost		321,826	314,472
Gross unrealized gains		14,750	6,926
Gross unrealized losses		(25)	(1,310)
Available-for-sale Securities, Debt Securities		336,551	320,088
Amortized Cost
Total amortized cost		321,826	314,472
Fair Value
Total fair value		$ 336,551	$ 320,088

[1] Mortgage-backed, asset-backed and commercial mortgage-backed

[2] Non-agency asset-backed and commercial mortgage-backed