Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 18, 2021
Document And Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Trading Symbol	ALTR
Entity Registrant Name	ALTAIR ENGINEERING INC.
Entity Central Index Key	0001701732
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity File Number	001-38263
Entity Tax Identification Number	38-2591828
Entity Address, Address Line One	1820 East Big Beaver Road
Entity Address, City or Town	Troy
Entity Address, State or Province	MI
Entity Address, Postal Zip Code	48083
City Area Code	248
Local Phone Number	614-2400
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Class A Common Stock $0.0001 par value per share
Security Exchange Name	NASDAQ
Class A Common Stock [Member]
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		50,789,524
Class B Common Stock [Member]
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		28,056,813