Loans - Summary of Allowance for Loan and Lease Losses for Loans Evaluated Individually and Collectively for Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment	$ 3,726	$ 2,377
Collectively evaluated for impairment	323,786	370,212
Total loans	327,512	372,589
Individually evaluated for impairment	31	12
Collectively evaluated for impairment	2,987	3,250
Total allowance for loan losses	3,018	3,262	$ 3,093
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment	2,497	1,165
Collectively evaluated for impairment	220,017	223,175
Total loans	222,514	224,340
Collectively evaluated for impairment	1,451	1,448
Total allowance for loan losses	1,451	1,448	1,369
Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment	1,170	1,176
Collectively evaluated for impairment	70,742	109,005
Total loans	71,912	110,181
Individually evaluated for impairment		6
Collectively evaluated for impairment	1,172	1,244
Total allowance for loan losses	1,172	1,250	1,246
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment	59	36
Collectively evaluated for impairment	33,027	38,032
Total loans	33,086	38,068
Individually evaluated for impairment	31	6
Collectively evaluated for impairment	364	558
Total allowance for loan losses	$ 395	$ 564	$ 478