Loans - Schedule of Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Receivables [Abstract]
Impaired loans without a valuation allowance	$ 2,461	$ 5,172	$ 5,134	$ 2,461	$ 5,134	$ 5,172
Impaired loans with a valuation allowance	9,388	9,360	10,125	9,388	10,125	9,360
Total impaired loans	11,849	14,532	15,259	11,849	15,259	14,532
Valuation allowance related to impaired loans	2,400	1,657	1,198	2,400	1,198	1,657
Total non-accrual loans	6,248	9,404	7,870	6,248	7,870	9,404
Total loans past-due ninety days or more and still accruing		0	0		0	0
Average investment in impaired loans	$ 11,639	$ 14,868	$ 13,517	$ 12,360	$ 8,613	$ 10,234

Amount of financing receivable 90 days or more past due and still accruing.

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