Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended
Mar. 31, 2019
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected original maturities period of marketable securities classified as available-for-sale	90 days

Represents the expected original maturity period of marketable securities classified as available-for-sale.

No definition available.

Name:	mack_AvailableForSaleSecuritiesExpectedMaturityPeriod
Namespace Prefix:	mack_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration