Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 16, 2019
Entity Registrant Name	B. RILEY PRINCIPAL MERGER CORP.
Entity Central Index Key	0001759824
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Trading Symbol	BRPM
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Reporting Status Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Document Fiscal Period Focus	Q1
Document Fiscal Year Focus	2019
Class A Common Stock [Member]
Entity Common Stock, Shares Outstanding		14,837,500
Class B Common Stock [Member]
Entity Common Stock, Shares Outstanding		3,593,750