Securities - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Investments [Abstract]
Available for sale securities carrying value pledged to secure public deposits	$ 2,800,000		$ 2,800,000		$ 17,300,000
Gross gains on sales of securities available for sale	0	$ 129,000	46,000	$ 294,000
Gross losses on sales of securities available for sale (less than in 2016)	0	$ 155,000	10,000	$ 243,000
Schedule Of Available-For-Sale And Held-To-Maturity [Line Items]
Fair value of available-for-sale securities	143,000,000		143,000,000		39,000,000
Unrealized loss of available-for-sale securities	$ 3,663,000		$ 3,663,000		$ 640,000
Number of securities with unrealized losses, held-to-maturity | security	3		3		1
Fair value of held-to-maturity securities	$ 1,274,000		$ 1,274,000		$ 254,000
Unrealized loss of held-to-maturity securities			$ 6,000		$ 1,000
U.S. government agencies
Schedule Of Available-For-Sale And Held-To-Maturity [Line Items]
Number of securities with unrealized losses, available-for-sale | security	18		18		3
Fair value of available-for-sale securities	$ 22,850,000		$ 22,850,000		$ 5,280,000
Unrealized loss of available-for-sale securities	$ 397,000		$ 397,000		$ 28,000
U.S. government-sponsored enterprises
Schedule Of Available-For-Sale And Held-To-Maturity [Line Items]
Number of securities with unrealized losses, available-for-sale | security	3		3		2
Fair value of available-for-sale securities	$ 6,042,000		$ 6,042,000		$ 3,469,000
Unrealized loss of available-for-sale securities	$ 16,000		$ 16,000		$ 1,000
State and political subdivisions
Schedule Of Available-For-Sale And Held-To-Maturity [Line Items]
Number of securities with unrealized losses, available-for-sale | security	48		48		9
Fair value of available-for-sale securities	$ 50,555,000		$ 50,555,000		$ 8,913,000
Unrealized loss of available-for-sale securities	$ 1,348,000		$ 1,348,000		$ 132,000
Number of securities with unrealized losses from state and political subdivisions maturity minimum number of years			10 years
Mortgage-backed securities - U.S. government-sponsored enterprises
Schedule Of Available-For-Sale And Held-To-Maturity [Line Items]
Number of securities with unrealized losses, available-for-sale | security	39		39		16
Fair value of available-for-sale securities	$ 61,566,000		$ 61,566,000		$ 21,338,000
Unrealized loss of available-for-sale securities	$ 1,889,000		$ 1,889,000		$ 479,000
Corporate debt
Schedule Of Available-For-Sale And Held-To-Maturity [Line Items]
Number of securities with unrealized losses, available-for-sale | security	1		1		0
Fair value of available-for-sale securities	$ 1,987,000		$ 1,987,000
Unrealized loss of available-for-sale securities	$ 13,000		$ 13,000
Maturity date (typically greater than)			5 years