Loans - Additional Information (Detail)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($) Contract	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans serviced for others		$ 86,200,000	$ 82,700,000
Concentrations of loans related to a single industry		0	0
Loans classified as troubled debt restructurings		2,300,000	2,900,000
Reserves allocated to customers whose loan terms are modified in troubled debt restructurings		23,000	35,000
Loans payable in nonaccrual of interest status		668,000
Mortgage loans in process of foreclosure amount		35,000	114,000
Other real estate real estate acquired through foreclosure		0	$ 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	$ 3,300,000
Loans restructured, subsequently defaulted | Contract	1
Performing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructurings		$ 1,600,000