| Document and Entity Information | Sep. 14, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001701732 |
| Document Type | 8-K |
| Document Period End Date | Sep. 14, 2020 |
| Entity Registrant Name | Altair Engineering Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38263 |
| Entity Tax Identification Number | 38-2591828 |
| Entity Address, Address Line One | 1820 E. 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Class A Common Stock $0.0001 par value per share |
| Trading Symbol | ALTR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
