Earnings Per Share (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
Earnings Per Share [Abstract]
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)
Net income applicable to common shareholders									$ 81,347	$ 15,509	$ 11,556
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
Effect of dilutive potential common shares:
Restricted stock units and warrants									1,082,700	677,249	901,397
Contingently issuable shares									45,421	150,302	208,119
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
Basic income per common share	$ 0.64	$ 1.29	$ 0.84	$ 0.31	$ (0.34)	$ 0.11	$ 0.67	$ 0.17	$ 3.08	$ 0.60	$ 0.50
Diluted income per common share	$ 0.59	$ 1.21	$ 0.82	$ 0.3	$ (0.34)	$ 0.1	$ 0.64	$ 0.17	$ 2.95	$ 0.58	$ 0.48