INVESTMENTS - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Available-for-sale
Total amortized cost		$ 1,745,648	$ 1,646,411
Available-for-sale Securities, Debt Securities		1,719,617	1,672,239
Amortized Cost
Due in one year or less		41,952
Due after one year through five years		379,636
Due after five years through 10 years		611,198
Due after 10 years		176,649
Mtge/ABS/CMBS	[1]	536,213
Total amortized cost		1,745,648	1,646,411
Fair Value
Due in one year or less		42,115
Due after one year through five years		378,403
Due after five years through 10 years		602,738
Due after 10 years		174,768
Mtge/ABS/CMBS	[1]	521,593
Total fair value		1,719,617	1,672,239
Fair value measured on recurring basis
Available-for-sale
Available-for-sale Securities, Debt Securities		1,719,617	1,672,239
Fair Value
Total fair value		1,719,617	1,672,239
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		1,745,648	1,646,411
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		9,120	34,216
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		35,151	8,388
Available-for-sale Securities, Debt Securities		1,719,617	1,672,239
Amortized Cost
Total amortized cost		1,745,648	1,646,411
Fair Value
Total fair value		1,719,617	1,672,239
U.S. government
Available-for-sale
Total amortized cost		177,339	92,561
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		8	23
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		3,560	895
Available-for-sale Securities, Debt Securities		173,787	91,689
Amortized Cost
Total amortized cost		177,339	92,561
Fair Value
Total fair value		173,787	91,689
U.S. Agency
Available-for-sale
Total amortized cost		30,647	18,541
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		111	347
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		855	110
Available-for-sale Securities, Debt Securities		29,903	18,778
Amortized Cost
Total amortized cost		30,647	18,541
Fair Value
Total fair value		29,903	18,778
Non-U.S. govt. & agency
Available-for-sale
Total amortized cost		8,178	7,501
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax			143
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		271	56
Available-for-sale Securities, Debt Securities		7,907	7,588
Amortized Cost
Total amortized cost		8,178	7,501
Fair Value
Total fair value		7,907	7,588
Mortgage-backed
Available-for-sale
Total amortized cost		415,476	329,129
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		1,299	3,420
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		14,749	4,078
Available-for-sale Securities, Debt Securities		402,026	328,471
Amortized Cost
Total amortized cost		415,476	329,129
Fair Value
Total fair value		402,026	328,471
ABS/CMBS
Available-for-sale
Total amortized cost	[2]	120,737	70,405
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[2]	113	436
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[2]	1,283	315
Available-for-sale Securities, Debt Securities	[2]	119,567	70,526
Amortized Cost
Total amortized cost	[2]	120,737	70,405
Fair Value
Total fair value	[2]	119,567	70,526
Corporate Debt
Available-for-sale
Total amortized cost		675,731	508,128
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		2,862	12,575
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		11,901	1,681
Available-for-sale Securities, Debt Securities		666,692	519,022
Amortized Cost
Total amortized cost		675,731	508,128
Fair Value
Total fair value		666,692	519,022
Municipal
Available-for-sale
Total amortized cost		317,540	620,146
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		4,727	17,272
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		2,532	1,253
Available-for-sale Securities, Debt Securities		319,735	636,165
Amortized Cost
Total amortized cost		317,540	620,146
Fair Value
Total fair value		$ 319,735	$ 636,165

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

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