CORPORATE DEBT - Schedule of Industry and Rate Sensitivity (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 1,011,275	[1]	$ 773,274
Aircraft and Parts
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	34,814		32,067
Coating, Engraving and Allied Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	64,034		0
Commercial Fishing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0		40,600
Computer Programming, Data Processing & Other Computer Related Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	192,946		146,547
Drugs
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	38,708		34,042
Electronic Components & Accessories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,916		0
Groceries and Related Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,794		14,856
Grocery Stores
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,531		23,761
Home Health Care Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,779		39,205
Insurance Agents, Brokers and Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	28,872		77,658
Management and Public Relations Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	94,871		16,493
Medical and Dental Laboratories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	26,956		17,292
Miscellaneous Business Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,723		84,486
Miscellaneous Equipment Rental and Leasing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	49,129		0
Miscellaneous Health and Allied Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,963		9,791
Miscellaneous Nonmetallic Minerals, except Fuels
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,992		24,688
Miscellaneous Plastic Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	9,879		27,036
Motor Vehicles and Motor Vehicle Parts and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,212		12,319
Offices and Clinics of Doctors of Medicine
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	76,678		83,386
Offices of Clinics and Other Health Practitioners
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	18,979		0
Personnel Supply Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0		36,921
Public Warehousing and Storage
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	48,890		0
Research, Development and Testing Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	33,155		17,744
Schools and Educational Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	20,625		20,979
Services Allied with the Exchange of Securities
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	13,960		0
Surgical, Medical, and Dental Instruments and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	29,687		13,403
Telephone Communications
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	59,182		0
Fixed Rate
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0		88,877
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 | Commercial Fishing
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 | Electronic Components & Accessories
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		4,391
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		84,486
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 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 of Clinics and Other Health Practitioners
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0		0
Fixed Rate | Personnel Supply Services
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	1,011,275		684,397
Floating Rate | Aircraft and Parts
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	34,814		32,067
Floating Rate | Coating, Engraving and Allied Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	64,034		0
Floating Rate | Commercial Fishing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0		40,600
Floating Rate | Computer Programming, Data Processing & Other Computer Related Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	192,946		146,547
Floating Rate | Drugs
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	38,708		34,042
Floating Rate | Electronic Components & Accessories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,916		0
Floating Rate | Groceries and Related Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,794		14,856
Floating Rate | Grocery Stores
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,531		23,761
Floating Rate | Home Health Care Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,779		39,205
Floating Rate | Insurance Agents, Brokers and Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	28,872		73,267
Floating Rate | Management and Public Relations Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	94,871		16,493
Floating Rate | Medical and Dental Laboratories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	26,956		17,292
Floating Rate | Miscellaneous Business Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,723		0
Floating Rate | Miscellaneous Equipment Rental and Leasing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	49,129		0
Floating Rate | Miscellaneous Health and Allied Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,963		9,791
Floating Rate | Miscellaneous Nonmetallic Minerals, except Fuels
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,992		24,688
Floating Rate | Miscellaneous Plastic Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	9,879		27,036
Floating Rate | Motor Vehicles and Motor Vehicle Parts and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,212		12,319
Floating Rate | Offices and Clinics of Doctors of Medicine
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	76,678		83,386
Floating Rate | Offices of Clinics and Other Health Practitioners
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	18,979		0
Floating Rate | Personnel Supply Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	0		36,921
Floating Rate | Public Warehousing and Storage
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	48,890		0
Floating Rate | Research, Development and Testing Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	33,155		17,744
Floating Rate | Schools and Educational Services, not elsewhere classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	20,625		20,979
Floating Rate | Services Allied with the Exchange of Securities
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	13,960		0
Floating Rate | Surgical, Medical, and Dental Instruments and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	29,687		13,403
Floating Rate | Telephone Communications
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 59,182		$ 0

[1] As a result of a change to a clearing organization’s rulebook effective January 3, 2017, beginning with the first quarter 2017 and in subsequent periods the Company is presenting the fair value of centrally cleared interest rate swaps net of variation margin pledged under such transactions. The variation margin was previously reported under cash and cash equivalents and is currently reported as a reduction to interest rate swaps, at fair value. Balances reported prior to the effective date will not be adjusted.