Loans - Recorded Investment in Residential, Consumer and Credit Card Loans Based on Payment Activity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	$ 771,309	$ 713,347
Consumer
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	86,035	80,193
Credit cards
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	7,623	6,753
Performing | Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	764,987	707,448
Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	85,308	79,328
Performing | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	7,618	6,699
Nonperforming | Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	6,322	5,899
Nonperforming | Consumer
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	727	865
Nonperforming | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Amount of recorded investment	$ 5	$ 54