DIVIDEND AND DISTRIBUTION (Details)	3 Months Ended
Apr. 30, 2019
DIVIDEND AND DISTRIBUTION
Period after the close of each calendar quarter when the fiscal quarter ends	1 month

Represents the period after the close of each calendar quarter when the fiscal quarter of the entity ends.

No definition available.

Name:	msb_PeriodAfterEachCalendarQuarterWhenEntityFiscalQuarterEnds
Namespace Prefix:	msb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration