Junior Subordinated Debt/Trust Preferred Securities (Details)		1 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2009 USD ($)	Sep. 30, 2015 USD ($)	Aug. 31, 2015 USD ($)	Jul. 31, 2007 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) quarter $ / shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jul. 31, 2013 USD ($)
Debt Instrument [Line Items]
Amount of junior subordinated debentures relating to trust preferred securities				$ 15,000,000		$ 12,000,000				$ 15,000,000
Maximum number of consecutive quarters the entity defer interest payments without default or penalty | quarter						20
Adoption of ASU 2016-01: reclassification of TRUPS to accumulated other comprehensive income						$ 1,482,000
Fair value, pretax loss adjustment					$ (424,000)	(392,000)	$ (882,000)
Fair value, loss adjustment net of tax					$ (301,000)	(278,000)	(519,000)
Fair value, gain on assets						32,000	0	$ 0
Fair value, gain on assets, net of tax						23,000
Junior Subordinated Debt
Debt Instrument [Line Items]
Amount of junior subordinated debentures relating to trust preferred securities	$ 15,000,000.0					12,000,000.0
Life of debt instrument				30 years	30 years
Debt instrument variable interest rate (percent)				1.29%
Initial coupon rate (percent)				6.65%
Frequency of interest payment	quarterly
Accrued and unpaid interest on debt						113,000	$ 80,000
Fair value discount (percentage)			40.00%
Amount of debt canceled		$ 3,000,000							$ 3,000,000
Gain on redemption of JR subordinated debentures		$ 78,000
Debentures						9,600,000
Cumulative gain recorded on debt						$ 2,400,000
United Security Bank | Junior Subordinated Debt
Debt Instrument [Line Items]
Amount of junior subordinated debentures relating to trust preferred securities			$ 3,000,000.0
Weighted average | Measurement Input, Discount Rate [Member] | Junior Subordinated Debt | Significant Unobservable Inputs Level 3
Debt Instrument [Line Items]
Weighted average inputs (percent)						0.0586
Accounting Standards Update 2016-01
Debt Instrument [Line Items]
Impact of ddoption on earnings per share, basic (in dollars per share) | $ / shares						$ 0.01
Impact of ddoption on earnings per share, diluted (in dollars per share) | $ / shares						$ 0.01