BUSINESS SEGMENTS (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Sep. 30, 2014
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Segment Reporting Information [Line Items]                        
Revenues - Services and fees                   $ 164,235 $ 101,929 $ 67,257
Revenues - Sale of goods                   26,116 10,596 9,859
Total revenues                   190,351 112,525 77,116
Selling, general, and administrative expenses                   (82,127) (58,322) (44,453)
Depreciation and amortization                   (4,306) (848) (646)
Restructuring costs                 $ 2,548 3,887 2,548
Segment income (loss) $ 31,458 $ 15,422 $ 180 $ 1,665 $ (1,326) $ 3,277 $ 14,669 $ 5,462   48,725 22,082 (7,431)
Consolidated operating income from reportable segments                   65,287 32,057 4,190
Corporate and other expenses                   (16,562) (9,975) (11,621)
Interest income                   318 17 12
Interest expense                   (1,996) (834) (1,262)
Income (loss) before income taxes                   47,047 21,265 (8,681)
(Provision) benefit for income taxes 8,137 6,083 (65) 166 (372) 600 5,684 1,775   (14,321) (7,688) 2,886
Net income (loss) 23,009 8,374 (27) 1,370 (1,046) 2,618 8,569 3,436   32,726 13,577 (5,795)
Net income attributable to noncontrolling interests                   11,200 1,772 6
Net income (loss) attributable to B. Riley Financial, Inc. $ 12,440 $ 8,939 $ (101) $ 248 $ (1,004) $ 1,463 $ 8,664 $ 2,682   21,526 11,805 (5,801)
Capital Markets Reportable Segment [Member]                        
Segment Reporting Information [Line Items]                        
Revenues - Services and fees                   39,335 35,183 19,420
Selling, general, and administrative expenses                   (32,695) (30,229) (14,185)
Depreciation and amortization                   (549) (519) (193)
Segment income (loss)                   6,091 4,435 5,042
Auction and Liquidation Reportable Segment [Member]                        
Segment Reporting Information [Line Items]                        
Revenues - Services and fees                   61,891 35,633 17,166
Revenues - Sale of goods                   25,855 10,596 9,859
Total revenues                   87,746 46,229 27,025
Direct cost of services                   (17,787) (15,489) (10,719)
Cost of goods sold                   (14,502) (3,072) (14,080)
Selling, general, and administrative expenses                   (14,331) (8,170) (8,481)
Depreciation and amortization                   (26) (191) (107)
Restructuring costs                   (1,339)
Segment income (loss)                   41,100 19,307 (7,701)
Valuation and Appraisal Reportable Segment [Member]                        
Segment Reporting Information [Line Items]                        
Revenues - Services and fees                   31,749 31,113 30,671
Direct cost of services                   (13,983) (13,560) (12,747)
Selling, general, and administrative expenses                   (8,778) (9,101) (10,721)
Depreciation and amortization                   (107) (137) (151)
Restructuring costs                   (203)
Segment income (loss)                   8,881 8,315 6,849
Principal Investments - United Online [Member]                        
Segment Reporting Information [Line Items]                        
Revenues - Services and fees                   31,260
Revenues - Sale of goods                   261
Total revenues                   31,521
Direct cost of services                   (9,087)
Cost of goods sold                   (253)
Selling, general, and administrative expenses                   (5,974)
Depreciation and amortization                   (3,518)    
Restructuring costs                   3,474
Segment income (loss)                   $ 9,215