Commitments and Contingencies - Schedule of FHLB Borrowings (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance	$ 33,000	$ 23,000
Rate	2.25%	1.35%
October 6, 2017 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance		$ 5,000
Rate		0.88%
Maturity		Oct. 06, 2017
July 6, 2018 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance		$ 6,000
Rate		1.18%
Maturity		Jul. 06, 2018
January 10, 2019 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance		$ 7,000
Rate		1.55%
Maturity		Jan. 10, 2019
January 10, 2020 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance	$ 5,000	$ 5,000
Rate	1.73%	1.73%
Maturity	Jan. 10, 2020	Jan. 10, 2020
January 10, 2019 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance	$ 7,000
Rate	1.55%
Maturity	Jan. 10, 2019
July 10, 2019 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance	$ 5,000
Rate	2.56%
Maturity	Jul. 10, 2019
July 10, 2020 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance	$ 5,000
Rate	2.84%
Maturity	Jul. 10, 2020
October 6, 2020 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance	$ 5,000
Rate	1.92%
Maturity	Oct. 06, 2020
July 6, 2023 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Balance	$ 6,000
Rate	3.01%
Maturity	Jul. 06, 2023