Loans - Number of Outstanding Commitments to Extend Credit (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commitments to Make Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Fixed Rate	$ 80,741	$ 74,927
Variable Rate	39,978	40,908
Undisbursed Loans in Process [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Fixed Rate	6,779	5,450
Variable Rate	163,903	130,566
Unused Lines of Credit [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Fixed Rate	9,503	8,538
Variable Rate	$ 247,891	$ 156,032