Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Net income (loss)	$ (1,866)	$ 635	$ 416	$ 404	$ (105)	$ 550	$ 462	$ 317	$ (411)	$ 1,224
Dividends paid									(486)	(483)
Net (decrease) increase in cash and cash equivalents									3,594	(20,542)
Cash and cash equivalents, beginning of period				23,530				44,072	23,530	44,072
Cash and cash equivalents, end of period	27,124				23,530				27,124	23,530
FUSB [Member]
Net income (loss)									(411)	1,224
Distributions in excess of undistributed income (loss) of subsidiaries									867	(763)
Change in other assets and liabilities									163	98
Net cash provided by operating activities									619	559
Dividends paid									(486)	(483)
Net cash used in financing activities									(486)	(483)
Net (decrease) increase in cash and cash equivalents									133	76
Cash and cash equivalents, beginning of period				$ 226				$ 150	226	150
Cash and cash equivalents, end of period	$ 359				$ 226				$ 359	$ 226