CORPORATE DEBT - Schedule of Industry and Rate Sensitivity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 1,256,276	$ 1,011,275	[1]
Aircraft and Parts
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	38,022	34,814
Coating, Engraving and Allied Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	60,049	64,034
Computer Programming, Data Processing & Other Computer Related Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	212,750	209,624
Drugs
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	38,730	38,708
Electrical Work
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	39,457	0
Electronic Components & Accessories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,995	23,916
Engineering, Architectural & Surveying
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	10,635	0
Groceries and Related Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,745	14,794
Grocery Stores
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,486	23,531
Home Health Care Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	23,779
Insurance Agents, Brokers and Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	49,480	28,872
Mailing, Reproduction, Commercial Art and Photography, and Stenographic
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,863	0
Management and Public Relations Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	210,511	94,871
Medical and Dental Laboratories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	26,904	26,956
Miscellaneous Business Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,677	19,723
Miscellaneous Equipment Rental and Leasing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	49,375	49,129
Miscellaneous Health and Allied Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	54,196	25,963
Miscellaneous Nonmetallic Minerals, except Fuels
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	25,992
Miscellaneous Plastic Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	9,937	9,879
Motor Vehicles and Motor Vehicle Equipment
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	17,138	0
Motor Vehicles and Motor Vehicle Parts and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,546	12,212
Offices and Clinics of Doctors of Medicine
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	97,722	60,000
Offices and Clinics of Other Health Practitioners
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	20,053	18,979
Public Warehousing and Storage
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	55,057	48,890
Research, Development and Testing Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	33,282	33,155
Schools and Educational Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,806	20,625
Services Allied with the Exchange of Securities
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,909	13,960
Surgical, Medical, and Dental Instruments and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	16,658	29,687
Telephone Communications
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	61,293	59,182
Fixed Rate
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Aircraft and Parts
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Coating, Engraving and Allied Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Computer Programming, Data Processing & Other Computer Related Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Drugs
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Electrical Work
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Electronic Components & Accessories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Engineering, Architectural & Surveying
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Groceries and Related Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Grocery Stores
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Home Health Care Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Insurance Agents, Brokers and Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Mailing, Reproduction, Commercial Art and Photography, and Stenographic
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Management and Public Relations Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Medical and Dental Laboratories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Miscellaneous Business Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Miscellaneous Equipment Rental and Leasing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Miscellaneous Health and Allied Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Miscellaneous Nonmetallic Minerals, except Fuels
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Miscellaneous Plastic Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Motor Vehicles and Motor Vehicle Equipment
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Motor Vehicles and Motor Vehicle Parts and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Offices and Clinics of Doctors of Medicine
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Offices and Clinics of Other Health Practitioners
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Public Warehousing and Storage
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Research, Development and Testing Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Schools and Educational Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Services Allied with the Exchange of Securities
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Surgical, Medical, and Dental Instruments and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Fixed Rate | Telephone Communications
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	0
Floating Rate
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	1,256,276	1,011,275
Floating Rate | Aircraft and Parts
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	38,022	34,814
Floating Rate | Coating, Engraving and Allied Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	60,049	64,034
Floating Rate | Computer Programming, Data Processing & Other Computer Related Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	212,750	209,624
Floating Rate | Drugs
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	38,730	38,708
Floating Rate | Electrical Work
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	39,457	0
Floating Rate | Electronic Components & Accessories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,995	23,916
Floating Rate | Engineering, Architectural & Surveying
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	10,635	0
Floating Rate | Groceries and Related Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,745	14,794
Floating Rate | Grocery Stores
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,486	23,531
Floating Rate | Home Health Care Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	23,779
Floating Rate | Insurance Agents, Brokers and Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	49,480	28,872
Floating Rate | Mailing, Reproduction, Commercial Art and Photography, and Stenographic
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,863	0
Floating Rate | Management and Public Relations Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	210,511	94,871
Floating Rate | Medical and Dental Laboratories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	26,904	26,956
Floating Rate | Miscellaneous Business Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,677	19,723
Floating Rate | Miscellaneous Equipment Rental and Leasing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	49,375	49,129
Floating Rate | Miscellaneous Health and Allied Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	54,196	25,963
Floating Rate | Miscellaneous Nonmetallic Minerals, except Fuels
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0	25,992
Floating Rate | Miscellaneous Plastic Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	9,937	9,879
Floating Rate | Motor Vehicles and Motor Vehicle Equipment
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	17,138	0
Floating Rate | Motor Vehicles and Motor Vehicle Parts and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,546	12,212
Floating Rate | Offices and Clinics of Doctors of Medicine
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	97,722	60,000
Floating Rate | Offices and Clinics of Other Health Practitioners
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	20,053	18,979
Floating Rate | Public Warehousing and Storage
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	55,057	48,890
Floating Rate | Research, Development and Testing Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	33,282	33,155
Floating Rate | Schools and Educational Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,806	20,625
Floating Rate | Services Allied with the Exchange of Securities
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,909	13,960
Floating Rate | Surgical, Medical, and Dental Instruments and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	16,658	29,687
Floating Rate | Telephone Communications
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 61,293	$ 59,182

[1] Derived from the audited consolidated financial statements at December 31, 2017.