Business Segments (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues - Services and fees									$ 566,956	$ 390,555	$ 304,841
Selling, general and administrative expenses									(385,219)	(310,508)	(227,884)
Restructuring recovery (charge)									(1,699)	(8,506)	(12,374)
Interest expense - Securities lending and loan participations sold									32,144	23,039	12,051
Depreciation and amortization									(19,048)	(13,809)	(11,140)
Segment income	$ 37,567	$ 59,851	$ 44,255	$ 24,978	$ (6,534)	$ 12,838	$ 28,478	$ 10,602	166,651	45,384	28,844
Cost of goods sold									7,575	800	398
(Loss) income on equity investments									(1,431)	7,986	(437)
Interest expense									(50,205)	(33,393)	(8,382)
Income before income taxes									116,592	21,303	20,445
Provision for income taxes	7,842	14,409	9,289	3,104	(3,509)	2,046	5,377	989	(34,644)	(4,903)	(8,510)
Net income	17,516	34,144	22,309	7,979	(8,965)	2,722	17,801	4,842	81,948	16,400	11,935
Net income attributable to noncontrolling interests									337	891	379
Net income attributable to B. Riley Financial, Inc.	$ 17,129	$ 34,302	$ 22,157	$ 8,023	$ (8,805)	$ 2,814	$ 16,997	$ 4,503	81,611	15,509	11,556
Preferred stock dividends									264
Auction and Liquidation segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees									18,296	54,923	47,376
Revenues - Sale of goods									4,220	63	3
Total revenues									22,516	54,986	47,379
Selling, general and administrative expenses									(10,730)	(8,274)	(8,329)
Depreciation and amortization									(7)	(31)	(21)
Segment income									(25,533)	27,013	11,186
Direct cost of services									(33,296)	(19,627)	(27,841)
Cost of goods sold									(4,016)	(41)	(2)
Capital Markets Segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees									408,637	236,789	169,967
Interest income - Loans and Securities lending									77,221	38,277	19,756
Total revenues									485,858	275,066	189,723
Selling, general and administrative expenses									(269,494)	(227,774)	(150,092)
Restructuring recovery (charge)									4	(8,378)	(7,855)
Interest expense - Securities lending and loan participations sold									(32,144)	(23,039)	(12,051)
Depreciation and amortization									(4,974)	(5,723)	(3,794)
Segment income									179,250	10,152	15,931
Valuation and Appraisal segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees									38,821	38,705	33,331
Selling, general and administrative expenses									(28,448)	(27,403)	(23,437)
Depreciation and amortization									(136)	(205)	(181)
Segment income									10,237	11,097	9,713
Principal Investments - United Online and magicJack segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees									97,147	53,659	51,439
Revenues - Sale of goods									3,715	575	304
Total revenues									100,862	54,234	51,743
Selling, general and administrative expenses									(24,256)	(10,962)	(11,304)
Restructuring recovery (charge)									(1,703)	(338)	(723)
Depreciation and amortization									(12,658)	(7,600)	(7,033)
Segment income									33,157	19,448	19,503
Direct cost of services									(25,529)	(15,127)	(12,784)
Cost of goods sold									(3,559)	(759)	(396)
Consolidated operating income from reportable segments									199,778	67,710	56,333
Corporate and other expenses (including restructuring recovery of $210 during the year ended December 31, 2018)									(33,127)	(22,326)	(27,489)
Interest income									1,577	1,326	420
(Loss) income on equity investments									(1,431)	7,986	(437)
Interest expense									(50,205)	(33,393)	(8,382)
Income before income taxes									116,592	21,303	20,445
Provision for income taxes									(36,644)	(4,903)	(8,510)
Net income									81,948	16,400	11,935
Net income attributable to noncontrolling interests									337	891	379
Net income attributable to B. Riley Financial, Inc.									81,611	15,509	11,556
Preferred stock dividends									$ 264
Net income available to common shareholders									$ 81,347	$ 15,509	$ 11,556
Brands Segment [Member]
Segment Reporting Information [Line Items]
Revenues - Services and fees									$ 4,055
Selling, general and administrative expenses									(881)
Depreciation and amortization									(507)
Segment income									$ 2,667