Selected Quarterly Financial Data (Unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 165,237	$ 180,063	$ 164,684	$ 142,128	$ 102,031	$ 99,681	$ 125,501	$ 95,778	$ 652,112	$ 422,991	$ 322,176
Operating income (loss)	37,567	59,851	44,255	24,978	(6,534)	12,838	28,478	10,602	166,651	45,384	28,844
Income (loss) before income taxes	25,358	48,553	31,598	11,083	(12,474)	4,768	23,178	5,831
(Provision for) benefit from 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 (loss)	17,516	34,144	22,309	7,979	(8,965)	2,722	17,801	4,842	81,948	16,400	11,935
Net income (loss) 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
Earnings (loss) per common share:
Basic	$ 0.64	$ 1.29	$ 0.84	$ 0.31	$ (0.34)	$ 0.11	$ 0.67	$ 0.17	$ 3.08	$ 0.60	$ 0.50
Diluted	$ 0.59	$ 1.21	$ 0.82	$ 0.3	$ (0.34)	$ 0.1	$ 0.64	$ 0.17	$ 2.95	$ 0.58	$ 0.48
Weighted average common shares outstanding:
Basic	26,547,023	26,556,223	26,278,352	26,217,215	26,177,560	25,968,997	25,424,178	26,219,277	26,401,036	25,937,305	23,181,388
Diluted	28,412,871	28,233,423	26,896,573	26,687,531	26,177,560	26,854,261	26,397,513	27,271,819	27,529,157	26,764,856	24,290,904