Net (Loss) Income Per Common Share - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Amounts allocated to participating securities	$ 0	$ 0
Participating securities outstanding, shares	0	0

Undistributed earnings (loss) allocated to participating securities.

No definition available.

Name:	mack_UndistributedEarningsLossAllocatedToParticipatingSecurities
Namespace Prefix:	mack_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration