Schedule of Industry and Rate Sensitivity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017		Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 856,110	[1]	$ 773,274	[2]
Aircraft and Parts
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	34,846		32,067
Coating, Engraving and Allied Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	63,643
Commercial Fishing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt			40,600
Computer Programming, Data Processing and Other Computer Related Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	149,005		146,547
Drugs
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	33,431		34,042
Electronic Components & Accessories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,885
Groceries and Related Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,803		14,856
Grocery Stores
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,560		23,761
Home Health Care Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,893		39,205
Insurance Agents, Brokers and Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	77,159		77,658
Management and Public Relations
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	94,608		16,493
Medical and Dental Laboratories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,990		17,292
Miscellaneous Business Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,754		84,486
Miscellaneous Equipment Rental and Leasing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,651
Miscellaneous Health and Allied Services, Not Elsewhere Classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,982		9,791
Miscellaneous Nonmetallic Minerals, Except Fuels
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,931		24,688
Miscellaneous Plastic Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	9,978		27,036
Motor Vehicles and Motor Vehicle Parts and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,230		12,319
Offices and Clinics of Doctors of Medicine
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	59,991		83,386
Offices and Clinics of Other Health Practitioners
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	7,397
Personnel Supply Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt			36,921
Public Warehousing and Storage
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	40,900
Research, Development and Testing Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	17,732		17,744
Schools and Educational Services, Not Elsewhere Classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	20,941		20,979
Surgical, Medical, and Dental Instruments and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,961		13,403
Telephone Communications
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	17,839
Fixed Rate
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	4,604		88,877
Fixed Rate | Insurance Agents, Brokers and Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	4,604		4,391
Fixed Rate | Miscellaneous Business Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt			84,486
Floating Rate
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	851,506		684,397
Floating Rate | Aircraft and Parts
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	34,846		32,067
Floating Rate | Coating, Engraving and Allied Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	63,643
Floating Rate | Commercial Fishing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt			40,600
Floating Rate | Computer Programming, Data Processing and Other Computer Related Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	149,005		146,547
Floating Rate | Drugs
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	33,431		34,042
Floating Rate | Electronic Components & Accessories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,885
Floating Rate | Groceries and Related Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	14,803		14,856
Floating Rate | Grocery Stores
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,560		23,761
Floating Rate | Home Health Care Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,893		39,205
Floating Rate | Insurance Agents, Brokers and Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	72,555		73,267
Floating Rate | Management and Public Relations
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	94,608		16,493
Floating Rate | Medical and Dental Laboratories
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,990		17,292
Floating Rate | Miscellaneous Business Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,754
Floating Rate | Miscellaneous Equipment Rental and Leasing
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,651
Floating Rate | Miscellaneous Health and Allied Services, Not Elsewhere Classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,982		9,791
Floating Rate | Miscellaneous Nonmetallic Minerals, Except Fuels
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	25,931		24,688
Floating Rate | Miscellaneous Plastic Products
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	9,978		27,036
Floating Rate | Motor Vehicles and Motor Vehicle Parts and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,230		12,319
Floating Rate | Offices and Clinics of Doctors of Medicine
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	59,991		83,386
Floating Rate | Offices and Clinics of Other Health Practitioners
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	7,397
Floating Rate | Personnel Supply Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt			36,921
Floating Rate | Public Warehousing and Storage
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	40,900
Floating Rate | Research, Development and Testing Services
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	17,732		17,744
Floating Rate | Schools and Educational Services, Not Elsewhere Classified
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	20,941		20,979
Floating Rate | Surgical, Medical, and Dental Instruments and Supplies
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,961		$ 13,403
Floating Rate | Telephone Communications
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 17,839

[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.

[2] Derived from the audited consolidated financial statements at December 31, 2016.