INVESTMENTS - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Available-for-sale
Total amortized cost		$ 1,772,097	$ 1,776,465
Available-for-sale Securities, Debt Securities		1,793,343	1,760,515
Amortized Cost
Due in one year or less		42,428
Due after one year through five years		390,764
Due after five years through 10 years		615,405
Due after 10 years		181,088
Mtge/ABS/CMBS	[1]	542,412
Total amortized cost		1,772,097	1,776,465
Fair Value
Due in one year or less		42,477
Due after one year through five years		395,761
Due after five years through 10 years		627,475
Due after 10 years		186,858
Mtge/ABS/CMBS	[1]	540,772
Total fair value		1,793,343	1,760,515
Fair value measured on recurring basis
Available-for-sale
Available-for-sale Securities, Debt Securities		1,793,343	1,760,515
Fair Value
Total fair value		1,793,343	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		1,772,097	1,776,465
Gross unrealized gains		31,873	13,539
Gross unrealized losses		(10,627)	(29,489)
Available-for-sale Securities, Debt Securities		1,793,343	1,760,515
Amortized Cost
Total amortized cost		1,772,097	1,776,465
Fair Value
Total fair value		1,793,343	1,760,515
U.S. government
Available-for-sale
Total amortized cost		192,148	199,982
Gross unrealized gains		2,964	1,232
Gross unrealized losses		(384)	(985)
Available-for-sale Securities, Debt Securities		194,728	200,229
Amortized Cost
Total amortized cost		192,148	199,982
Fair Value
Total fair value		194,728	200,229
U.S. Agency
Available-for-sale
Total amortized cost		31,712	31,716
Gross unrealized gains		1,043	403
Gross unrealized losses		(17)	(215)
Available-for-sale Securities, Debt Securities		32,738	31,904
Amortized Cost
Total amortized cost		31,712	31,716
Fair Value
Total fair value		32,738	31,904
Non-U.S. govt. & agency
Available-for-sale
Total amortized cost		8,162	8,170
Gross unrealized gains		74
Gross unrealized losses		(293)	(531)
Available-for-sale Securities, Debt Securities		7,943	7,639
Amortized Cost
Total amortized cost		8,162	8,170
Fair Value
Total fair value		7,943	7,639
Mortgage-backed
Available-for-sale
Total amortized cost		396,248	402,992
Gross unrealized gains		3,211	1,709
Gross unrealized losses		(5,493)	(9,448)
Available-for-sale Securities, Debt Securities		393,966	395,253
Amortized Cost
Total amortized cost		396,248	402,992
Fair Value
Total fair value		393,966	395,253
ABS/CMBS
Available-for-sale
Total amortized cost	[2]	146,164	137,224
Gross unrealized gains	[2]	983	375
Gross unrealized losses	[2]	(341)	(876)
Available-for-sale Securities, Debt Securities	[2]	146,806	136,723
Amortized Cost
Total amortized cost	[2]	146,164	137,224
Fair Value
Total fair value	[2]	146,806	136,723
Corporate Debt
Available-for-sale
Total amortized cost		684,492	681,909
Gross unrealized gains		12,934	2,894
Gross unrealized losses		(3,910)	(16,124)
Available-for-sale Securities, Debt Securities		693,516	668,679
Amortized Cost
Total amortized cost		684,492	681,909
Fair Value
Total fair value		693,516	668,679
Municipal
Available-for-sale
Total amortized cost		313,171	314,472
Gross unrealized gains		10,664	6,926
Gross unrealized losses		(189)	(1,310)
Available-for-sale Securities, Debt Securities		323,646	320,088
Amortized Cost
Total amortized cost		313,171	314,472
Fair Value
Total fair value		$ 323,646	$ 320,088

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

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