Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Nov. 10, 2020
Entity Registrant Name	B. Riley Principal Merger Corp. II
Entity Central Index Key	0001805077
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2020
Current Fiscal Year End Date	--12-31
Entity File Number	001-39291
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Extransition Period	false
Entity Shell Company	true
Entity Incorporation, State or Country Code	DE
Class A Common Stock
Entity Common Stock, Shares Outstanding		18,150,000
Class B Common Stock
Entity Common Stock, Shares Outstanding		4,375,000