Document and Entity Information - USD ($)	10 Months Ended
	Dec. 31, 2019	Mar. 17, 2020	Jun. 30, 2019
Entity Listings [Line Items]
Entity Registrant Name	South Mountain Merger Corp.
Entity Central Index Key	0001774155
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	true
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Public Float			$ 0
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Address, State or Province	NY
Shares of Class A common stock [Member]
Entity Listings [Line Items]
Entity Common Stock, Shares Outstanding		25,000,000
Shares of Class B common stock [Member]
Entity Listings [Line Items]
Entity Common Stock, Shares Outstanding		6,250,000