INVESTMENTS - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale
Total amortized cost	$ 1,881,963	$ 1,776,465
Available-for-sale Securities, Debt Securities	1,957,025	1,760,515
Amortized Cost
Due in one year or less	87,635
Due after one year through five years	397,448
Due after five years through 10 years	593,686
Due after 10 years	208,277
Mtge/ABS/CMBS	594,917
Total amortized cost	1,881,963	1,776,465
Fair Value
Due in one year or less	87,812
Due after one year through five years	408,691
Due after five years through 10 years	627,483
Due after 10 years	224,975
Mtge/ABS/CMBS	608,064
Total fair value	1,957,025	1,760,515
Fair value measured on recurring basis
Available-for-sale
Available-for-sale Securities, Debt Securities	1,957,025	1,760,515
Fair Value
Total fair value	1,957,025	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	1,881,963	1,776,465
Gross unrealized gains	77,562	13,539
Gross unrealized losses	(2,500)	(29,489)
Available-for-sale Securities, Debt Securities	1,957,025	1,760,515
Amortized Cost
Total amortized cost	1,881,963	1,776,465
Fair Value
Total fair value	1,957,025	1,760,515
U.S. government
Available-for-sale
Total amortized cost	232,269	199,982
Gross unrealized gains	8,705	1,232
Gross unrealized losses	(61)	(985)
Available-for-sale Securities, Debt Securities	240,913	200,229
Amortized Cost
Total amortized cost	232,269	199,982
Fair Value
Total fair value	240,913	200,229
U.S. Agency
Available-for-sale
Total amortized cost	29,703	31,716
Gross unrealized gains	2,776	403
Gross unrealized losses		(215)
Available-for-sale Securities, Debt Securities	32,479	31,904
Amortized Cost
Total amortized cost	29,703	31,716
Fair Value
Total fair value	32,479	31,904
Non-U.S. govt. & agency
Available-for-sale
Total amortized cost	9,241	8,170
Gross unrealized gains	290
Gross unrealized losses	(189)	(531)
Available-for-sale Securities, Debt Securities	9,342	7,639
Amortized Cost
Total amortized cost	9,241	8,170
Fair Value
Total fair value	9,342	7,639
Mortgage-backed
Available-for-sale
Total amortized cost	421,448	402,992
Gross unrealized gains	10,632	1,709
Gross unrealized losses	(571)	(9,448)
Available-for-sale Securities, Debt Securities	431,509	395,253
Amortized Cost
Total amortized cost	421,448	402,992
Fair Value
Total fair value	431,509	395,253
ABS/CMBS
Available-for-sale
Total amortized cost	173,469	137,224
Gross unrealized gains	3,230	375
Gross unrealized losses	(144)	(876)
Available-for-sale Securities, Debt Securities	176,555	136,723
Amortized Cost
Total amortized cost	173,469	137,224
Fair Value
Total fair value	176,555	136,723
Corporate Debt
Available-for-sale
Total amortized cost	675,463	681,909
Gross unrealized gains	34,148	2,894
Gross unrealized losses	(1,509)	(16,124)
Available-for-sale Securities, Debt Securities	708,102	668,679
Amortized Cost
Total amortized cost	675,463	681,909
Fair Value
Total fair value	708,102	668,679
Municipal
Available-for-sale
Total amortized cost	340,370	314,472
Gross unrealized gains	17,781	6,926
Gross unrealized losses	(26)	(1,310)
Available-for-sale Securities, Debt Securities	358,125	320,088
Amortized Cost
Total amortized cost	340,370	314,472
Fair Value
Total fair value	$ 358,125	$ 320,088

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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