Loans - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) Industry Contract Property	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) Loan Industry Contract Property	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) Loan Contract Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans serviced for others	$ 137,500,000		$ 137,500,000		$ 117,500,000
Interest and fee income on loans	6,897,000	$ 7,190,000	19,993,000	$ 21,145,000
Concentration loans	546,444,000		546,444,000		609,870,000
Assisted living facilities loans	538,450,000		538,450,000		600,885,000
Loans classified as troubled debt restructurings	2,600,000		2,600,000		2,800,000
Reserves allocated to customers whose loan terms are modified in troubled debt restructurings	25,000		25,000		$ 30,000
Loans payable classified as nonaccrual	$ 108,000		$ 108,000
Loans restructured, subsequently defaulted | Contract	0		0		0
Number of properties acquired | Property	0		0		0
Mortgage loans in process of foreclosure amount			$ 0		$ 21,000
Other repossessed assets	$ 33,000		33,000		$ 0
Outstanding balance of commercial loans classified under credit risk, minimum amount	500,000		500,000
Loans listed as not rated under risk category, maximum amount	500,000		500,000
Performing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructurings	2,500,000		2,500,000
Covid-19 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance including commitments	51,800,000		51,800,000
Business loans	33,500,000		33,500,000
Covid-19 [Member] | Assisted Living Facilities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Assisted living facilities loans	$ 14,000,000.0		$ 14,000,000.0
Credit Concentration Risk [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of industries for credit concentrations | Industry	2		2
Credit Concentration Risk [Member] | Loans Receivable [Member] | Lessors of Non-residential Buildings or Dwellings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration loans	$ 50,400,000		$ 50,400,000
Concentration of credit percentage of total loans			9.00%
Credit Concentration Risk [Member] | Loans Receivable [Member] | Assisted Living Facilities for Elderly [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration loans	33,500,000		$ 33,500,000
Concentration of credit percentage of total loans			6.00%
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of PPP loans | Loan			208		671
Principal amount outstanding on PPP loans	$ 17,000,000.0		$ 17,000,000.0		$ 70,100,000
PPP loans guaranteed percentage	100.00%		100.00%
Loan processing fee received			$ 5,400,000
Remaining unearned fees on loans outstanding	$ 620,000		620,000
Interest and fee income on loans	$ 969,000	$ 845,000	$ 2,700,000	$ 1,500,000