Schedule of Industry and Interest Rate Characteristics (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	[1]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 669,612	$ 488,508
Business Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	41,311
Commercial Fishing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	41,196
Cyber Security [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	93,517
Drugs [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	34,452
Home Health Care Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	39,455
Insurance Agents Brokers And Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	48,815
Miscellaneous Business Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,612
Miscellaneous Food Preparations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	27,221
Miscellaneous Health & Allied Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	39,031
Miscellaneous Nonmetallic Minerals [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	24,676
Miscellaneous Plastic Products [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	27,114
Motor Vehicles, Parts And Supplies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,375
Offices And Clinics of Doctors of Medicine [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	83,919
Research, Development And Testing Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	17,740
Schools And Educational Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	21,105
Security [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	84,410
Surgical, Medical And Dental Instruments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	13,663
Fixed Rate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	88,779
Fixed Rate [Member] | Insurance Agents Brokers And Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	4,369
Fixed Rate [Member] | Security [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	84,410
Floating Rate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	580,833
Floating Rate [Member] | Business Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	41,311
Floating Rate [Member] | Commercial Fishing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	41,196
Floating Rate [Member] | Cyber Security [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	93,517
Floating Rate [Member] | Drugs [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	34,452
Floating Rate [Member] | Home Health Care Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	39,455
Floating Rate [Member] | Insurance Agents Brokers And Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	44,446
Floating Rate [Member] | Miscellaneous Business Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	19,612
Floating Rate [Member] | Miscellaneous Food Preparations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	27,221
Floating Rate [Member] | Miscellaneous Health & Allied Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	39,031
Floating Rate [Member] | Miscellaneous Nonmetallic Minerals [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	24,676
Floating Rate [Member] | Miscellaneous Plastic Products [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	27,114
Floating Rate [Member] | Motor Vehicles, Parts And Supplies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,375
Floating Rate [Member] | Offices And Clinics of Doctors of Medicine [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	83,919
Floating Rate [Member] | Research, Development And Testing Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	17,740
Floating Rate [Member] | Schools And Educational Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	21,105
Floating Rate [Member] | Surgical, Medical And Dental Instruments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 13,663

[1] Derived from the audited consolidated financial statements at December 31, 2015.