| Cover | May 17, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | May 17, 2021 |
| Entity File Number | 001-36576 |
| Entity Registrant Name | Marinus Pharmaceuticals, Inc. |
| Entity Central Index Key | 0001267813 |
| Entity Tax Identification Number | 20-0198082 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 5 Radnor Corporate Center |
| Entity Address, Address Line Two | Suite 500 |
| Entity Address, Address Line Three | 100 Matsonford Rd |
| Entity Address, City or Town | Radnor |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 19087 |
| City Area Code | 484 |
| Local Phone Number | 801-4670 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 |
| Trading Symbol | MRNS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
