Related Party Transactions (Details) - USD ($)			1 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	May 14, 2020	Apr. 02, 2019	May 31, 2020	May 22, 2020	Jan. 31, 2020	Oct. 31, 2017	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Oct. 28, 2020
Related Party Transactions (Details) [Line Items]
Due from related party								$ 3,766,000		$ 5,832,000
Management fee earned						$ 200,000				13,000
Interest expense								45,543,000	$ 30,181,000
Transfer Agreement, description		the Company entered into a Transfer Agreement (the “Transfer Agreement”) with GACP II, a fund managed by GACP, and John Ahn, who is the brother of Phil Ahn, the Company’s Chief Financial Officer and Chief Operating Officer. The Transfer Agreement provides for among other things, the transfer to Mr. J. Ahn of 55.56% of the Company’s limited partnership interest in GACP II (the “Transferred Interest”), which represents a capital commitment in the aggregate amount of $5,000. In connection with the Transfer Agreement, the Company provided Mr. J. Ahn with a non-recourse, secured line of credit in an aggregate amount of up to $5,003 pursuant to the terms of a Secured Line of Credit Promissory Note (the “Note”) dated April 1, 2019, to fund the purchase price of the Transferred Interest. We also entered into a Security Agreement with Mr. J. Ahn on April 1, 2019, which granted to the Company a security interest in the Transferred Interest to secure Mr. J. Ahn’s obligations under the Note. The Note is subject to an interest rate per annum of 7.00%. As of December 31, 2019, the principal and accrued interest on the Note were $3,798 and $48, respectively. In June 2020, the Company entered into an investment advisory services agreement with Whitehawk Capital Partners, L.P., a limited partnership controlled by Mr. J. Ahn, (“Whitehawk”). Whitehawk has agreed to provide investment advisory services for GACP I and GACP II.  In accordance with the terms of the Note, Mr. Ahn surrendered the Transferred Interest to the Company in exchange for the cancellation of the Note.
Interest payments received								121,000
Management fees paid								$ 731,000
Underwriting fees				$ 3,275,000
Initial public offering, description				The Company has also agreed to loan BRPM II up to $300 for operating expenses. The loan is interest free and there were no amounts outstanding at September 30, 2020. On September 7, 2020, BRPM II entered into an agreement and plan of merger (the “Merger Agreement”) to acquire Eos Energy Storage LLC, a Delaware limited liability company, a privately held company that is not related to the Company (the “Proposed Acquisition”).
Merger agreement description								In order to help meet the condition under the Merger Agreement that BRPM II maintain a certain level of cash available upon the closing (before taking into account certain transaction expenses), the Company entered into an Equity Commitment Letter with BRPM II and B. Riley Principal Sponsor Co. II, LLC, pursuant to which the Company committed to provide up to $40,000 in equity financing at closing, less the number of shares of BRPM II’s common stock already issued pursuant to subscription agreements entered into with investors prior to the closing.
Services fees								$ 750,000
Debt issue discount								$ 10,036,000		8,875,000
Conversion on notes receivable											$ 3,367
Notes receivable from related party											$ 3,367
Executive Officer's and Board of Directors [Member]
Related Party Transactions (Details) [Line Items]
Financial interest								43.80%
GACP [Member]
Related Party Transactions (Details) [Line Items]
Loans receivable with a carrying value								$ 1,800,000
Sonim [Member]
Related Party Transactions (Details) [Line Items]
Loan amount							$ 6,170,000
Maven [Member]
Related Party Transactions (Details) [Line Items]
Principal amount due on loan										49,921,000
GACP I, L.P [Member]
Related Party Transactions (Details) [Line Items]
Due from related party								$ 23,000		145,000
GACP I, L.P [Member] | Co-Chief Executive Officer [Member]
Related Party Transactions (Details) [Line Items]
Financial interest								38.50%
GACP II, L.P [Member]
Related Party Transactions (Details) [Line Items]
Due from related party								$ 372,000		12,000
CA Global Partners, LLC [Member]
Related Party Transactions (Details) [Line Items]
Due from related party								3,371,000
President [Member]
Related Party Transactions (Details) [Line Items]
Due from related party								3,846,000
GACP I, L.P [Member]
Related Party Transactions (Details) [Line Items]
Subsidiaries amount value								13,919,000
Interest expense								1,416,000
GACP I, L.P [Member] | B. Riley Partners Opportunity Fund's Loan Participations [Member]
Related Party Transactions (Details) [Line Items]
Outstanding loan								$ 13,919,000		12,478,000
Sonim [Member]
Related Party Transactions (Details) [Line Items]
Loans receivable with a carrying value										9,603,000
Financial interest								10.00%
Related party transaction, date								Sep. 01, 2022
Principal amount due on loan							4,000,000
Babcock and Wilcox [Member]
Related Party Transactions (Details) [Line Items]
Loans receivable with a carrying value										109,147,000
Outstanding loan								$ 13,919,000
Loans receviable amount								$ 164,539,000
Additional loan amount					$ 30,000,000
Related party transaction, description	the Company provided B&W with another $30,000 of last-out term loans pursuant to a further amendment to B&W’s credit agreement which also included future commitments for the Company to loan B&W $40,000 at various dates starting in November 2020 and a limited guaranty by the Company of B&W’s obligations under the amended credit facility, (the “Amendment Transactions”). Interest is payable quarterly at the fixed rate of 12.0% per annum in common stock of B&W at $2.28 per common share through December 31, 2020 and in cash thereafter. All of these loans were made to B&W as part of various amendments to B&W’s existing credit agreement with other lenders not related to the Company.
Warrants granted (in Shares)								1,666,667
Exercise Price (in Dollars per share)								$ 0.01
Maturity date								Apr. 05, 2022
Maven [Member]
Related Party Transactions (Details) [Line Items]
Due from related party										21,150,000
Loans receivable with a carrying value								$ 71,479,000		47,933,000
Debt issue discount										$ 1,988,000
Description of loan payables										Interest on these loans is payable at 12.0% to 15.0% per annum with maturity dates through June 2022.
Franchise Group [Member]
Related Party Transactions (Details) [Line Items]
Loans receivable with a carrying value										$ 4,951,000
Financial interest										13.70%
Underwriting fees							$ 4,329,000
Maturity date										Dec. 16, 2022
Principal balance			$ 4,697,000
Interest payment			$ 31,000