BUSINESS SEGMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Revenues - Services and fees	$ 64,395	$ 20,261	$ 117,213	$ 40,205
Revenues - Sale of goods	63		142	2
Interest income - Securities lending	2,218		2,218
Total revenues	66,676	20,261	119,573	40,207
Interest expense - Securities lending	1,565		1,565
Selling, general, and administrative expenses	(37,722)	(14,521)	(61,874)	(26,117)
Depreciation and amortization			(4,290)	(399)
Restructuring costs	6,214		6,588
Segment (loss) income	2,560	180	13,271	1,845
Consolidated operating income from reportable segments	9,811	3,247	24,447	6,899
Corporate and other expenses (including restructuring costs of $2,182 for the three and six months ended June 30, 2017)	(7,251)	(3,067)	(11,176)	(5,054)
Interest income	150	3	282	6
Interest expense	(1,894)	(275)	(2,685)	(407)
Income (loss) before income taxes	816	(92)	10,868	1,444
Benefit (provision) for income taxes	2,547	65	6,396	(101)
Net income (loss)	3,363	(27)	17,264	1,343
Net income (loss) attributable to noncontrolling interests	83	74	(37)	1,196
Net income (loss) attributable to B. Riley Financial, Inc.	3,280	(101)	17,301	147
Corporate and other expenses, restructuring costs	2,182		2,182
Capital Markets Reportable Segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees	21,676	7,172	39,399	12,736
Interest income - Securities lending	2,218		2,218
Total revenues	23,894	7,172	41,617	12,736
Interest expense - Securities lending	(1,565)		(1,565)
Selling, general, and administrative expenses	(23,067)	(7,669)	(34,036)	(13,843)
Depreciation and amortization	(166)	(23)	(293)	(44)
Restructuring costs	(3,923)		(3,923)
Segment (loss) income	(4,827)	(520)	1,800	(1,151)
Auction and Liquidation Reportable Segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees	21,807	5,393	35,803	12,300
Revenues - Sale of goods				2
Total revenues	21,807	5,393	35,803	12,302
Direct cost of services	(11,763)	(2,087)	(22,097)	(5,505)
Cost of goods sold				(2)
Selling, general, and administrative expenses	(2,749)	(1,577)	(4,599)	(2,802)
Depreciation and amortization	(5)	(37)	(10)	(78)
Segment (loss) income	7,290	1,692	9,097	3,915
Valuation and Appraisal Reportable Segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees	7,960	7,696	15,756	15,169
Direct cost of services	(3,581)	(3,473)	(7,253)	(6,738)
Selling, general, and administrative expenses	(2,062)	(2,124)	(4,142)	(4,243)
Depreciation and amortization	(43)	(24)	(87)	(53)
Segment (loss) income	2,274	2,075	4,274	4,135
Principal Investments - United Online Segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees	12,952		26,255
Revenues - Sale of goods	63		142
Total revenues	13,015		26,397
Direct cost of services	(3,141)		(6,736)
Cost of goods sold	(130)		(189)
Selling, general, and administrative expenses	(2,791)		(6,103)
Depreciation and amortization	(1,770)		(3,610)
Restructuring costs	(109)		(483)
Segment (loss) income	$ 5,074		$ 9,276