Schedule of Industry and Interest Rate Characteristics (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	[1]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 639,481	$ 488,508
Healthcare [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	173,896
Security Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	74,700
Technology [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	93,292
Business Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	138,874
Agriculture And Protein Processing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	41,576
Food And Beverage [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	27,930
Mining And Minerals [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	24,671
Consumer Products [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,230
Manufacturing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	7,477
Education [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	21,168
Packaging [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	12,667
Fixed Rate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	79,059
Fixed Rate [Member] | Security Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	74,700
Fixed Rate [Member] | Business Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	4,359
Floating Rate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	560,422
Floating Rate [Member] | Healthcare [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	173,896
Floating Rate [Member] | Technology [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	93,292
Floating Rate [Member] | Business Services [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	134,515
Floating Rate [Member] | Agriculture And Protein Processing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	41,576
Floating Rate [Member] | Food And Beverage [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	27,930
Floating Rate [Member] | Mining And Minerals [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	24,671
Floating Rate [Member] | Consumer Products [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	23,230
Floating Rate [Member] | Manufacturing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	7,477
Floating Rate [Member] | Education [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	21,168
Floating Rate [Member] | Packaging [Member]
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt	$ 12,667

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