Investment Securities - Summary of Securities with Gross Unrealized Losses (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Losses, Less than Twelve Months	$ (285)
Fair Value, Less than Twelve Months	13,083
Gross Unrealized Losses, Twelve Months and Over	(1,354)
Fair Value, Twelve Months and Over	30,477
Mortgage-backed Securities, Principally Obligations of US Federal Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Losses, Less than Twelve Months	(143)
Fair Value, Less than Twelve Months	7,165
Gross Unrealized Losses, Twelve Months and Over	(1,045)
Fair Value, Twelve Months and Over	24,751
State and Municipalities [Member]
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Losses, Less than Twelve Months	(142)
Fair Value, Less than Twelve Months	5,918
Gross Unrealized Losses, Twelve Months and Over	(309)
Fair Value, Twelve Months and Over	$ 5,726