| Document and Entity Information | Feb. 18, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 18, 2021 |
| Entity Registrant Name | Casa Systems, Inc |
| Entity Central Index Key | 0001333835 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity File Number | 001-38324 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 75-3108867 |
| Entity Address, Address Line One | 100 Old River Road |
| Entity Address, City or Town | Andover |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 01810 |
| City Area Code | (978) |
| Local Phone Number | 688-6706 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | CASA |
| Security Exchange Name | NASDAQ |
