Securities - Trading Account Securities, at Estimated Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 24,495	$ 24,298
U.S. Treasury [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading account securities	$ 24,495	$ 24,298

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_TradingSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant