Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2017
Summary of Significant Accounting Policies
Sale of research and development tax credits to third party	$ 0.4	$ 0.4

Represents proceeds from sale of R&D tax credits to a third party.

No definition available.

Name:	mrns_ProceedsFromSaleOfResearchAndDevelopmentTaxCredits
Namespace Prefix:	mrns_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration