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STATEMENTS OF UNALLOCATED RESERVE AND TRUST CORPUS (Parenthetical) - $ / shares | 12 Months Ended
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Jan. 31, 2019 | Jan. 31, 2018 | Jan. 31, 2017
STATEMENTS OF UNALLOCATED RESERVE AND TRUST CORPUS |  |  |
First Quarterly Distribution Paid Per Unit | $ 0.45 | $ 0.55 | $ 0.20
Second Quarterly Distribution Paid Per Unit | 0.22 | 0.16 | 0.20
Third Quarterly Distribution Paid Per Unit | 0.94 | 0.64 | 0.10
Fourth Quarterly Distribution Declared Or Paid Per Unit | $ 1.39 | $ 1.18 | $ 0.14

X
- Definition Amount per unit paid out as the first quarterly distribution for the reporting period. + References No definition available. + Details
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Namespace Prefix: | msb_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount per unit declared or paid out as the fourth quarterly distribution for the reporting period. + References No definition available. + Details
Name: | msb_FourthQuarterlyDistributionDeclaredOrPaidPerUnit
Namespace Prefix: | msb_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount per unit paid out as the second quarterly distribution for the reporting period. + References No definition available. + Details
Name: | msb_SecondQuarterlyDistributionPaidPerUnit
Namespace Prefix: | msb_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
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Namespace Prefix: | msb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount per unit paid out as the third quarterly distribution for the reporting period. + References No definition available. + Details
Name: | msb_ThirdQuarterlyDistributionPaidPerUnit
Namespace Prefix: | msb_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration