Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 11, 2019	Jun. 30, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	MARINUS PHARMACEUTICALS INC
Entity Central Index Key	0001267813
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 265,327,966
Entity Common Stock, Shares Outstanding		52,519,013
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY