Leases - Components of Lease Expense (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease expense	[1]	$ 841	$ 839
Operating lease income	[2]	842	845
Operating lease right-of-use assets		$ 3,070
Operating Lease, Liability, Statement of Financial Position [Extensible List]		fusb:OperatingLeaseLiabilitiesMember
Operating lease liabilities		$ 3,125
Weighted-average remaining lease term (in years)		5 years 9 months 14 days
Weighted-average discount rate		3.06%
Operating cash flows from operating leases		$ 733	$ 764

[1] Includes short-term lease costs. For the years ended December 31, 2020 and 2019, short-term lease costs were nominal in amount.

[2] Operating lease income includes rental income from owned properties.