Loans - Additional Information (Detail)	3 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2016 USD ($) Contract	Sep. 30, 2017 USD ($)	Sep. 30, 2016 Contract	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans serviced for others		$ 79,800,000		$ 85,900,000
Concentrations of loans related to a single industry	$ 0	0		0
Loans classified as troubled debt restructurings	6,400,000	3,100,000		6,400,000
Reserves allocated to customers whose loan terms are modified in troubled debt restructurings	711,000	24,000		711,000
Loans payable in nonaccrual of interest status		1,100,000
Foreclosed real estate	$ 0	0		0
Mortgage loans in process of foreclosure amount		154,000		$ 448,000
Loans restructured, subsequently defaulted | Contract			0
Outstanding balance of commercial loans classified under credit risk, minimum amount		300,000
Loans listed as not rated under risk category, maximum amount		300,000
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans restructured, subsequently defaulted | Contract	1
Loans restructured, subsequently defaulted	$ 3,300,000
Performing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructurings		$ 2,000,000