Information for Financial Instruments on Which Fair Value Option was Elected (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Option, Quantitative Disclosures [Line Items]
Excess (Deficiency) of fair value over uncollected principal	$ (124,359)	$ (325,324)
Debt Securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Excess (Deficiency) of fair value over uncollected principal	2,697	(26,660)
Corporate Loans
Fair Value, Option, Quantitative Disclosures [Line Items]
Excess (Deficiency) of fair value over uncollected principal	(126,907)	(301,085)
Loans and Receivables
Fair Value, Option, Quantitative Disclosures [Line Items]
Excess (Deficiency) of fair value over uncollected principal	$ (149)	$ 2,421