Securities - Trading Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	$ 8,887	$ 7,150
Corporate equity securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	6,643	5,125
Mutual funds
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	1,687	1,499
Corporate notes and bonds
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	278	254
U.S. gov’t sponsored entities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	51	52
Certificates of deposit
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	$ 228	$ 220