Fair Value - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Detail) - Fair Value, Measurements, Non Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Commercial, industrial, and agricultural
Impaired loans:
Commercial mortgages	$ 783
Commercial, industrial, and agricultural | Quoted Prices in Active Markets for Identical Assets (Level 1)
Impaired loans:
Commercial mortgages	0
Commercial, industrial, and agricultural | Significant Other Observable Inputs (Level 2)
Impaired loans:
Commercial mortgages	0
Commercial, industrial, and agricultural | Significant Unobservable Inputs (Level 3)
Impaired loans:
Commercial mortgages	783
Commercial mortgages
Impaired loans:
Commercial mortgages	224	$ 11
Commercial mortgages | Quoted Prices in Active Markets for Identical Assets (Level 1)
Impaired loans:
Commercial mortgages	0	0
Commercial mortgages | Significant Other Observable Inputs (Level 2)
Impaired loans:
Commercial mortgages	0	0
Commercial mortgages | Significant Unobservable Inputs (Level 3)
Impaired loans:
Commercial mortgages	$ 224	$ 11