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STATEMENTS OF UNALLOCATED RESERVE AND TRUST CORPUS (Parenthetical) - $ / shares | 12 Months Ended
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Jan. 31, 2022 | Jan. 31, 2021 | Jan. 31, 2020
First quarterly distribution paid per unit (in dollars per unit) | $ 0.89 |  | $ 0.89
Second quarterly distribution paid per unit (in dollars per unit) | 0.09 | $ 0.05 | 0.21
Third quarterly distribution paid per unit (in dollars per unit) | 1.42 | 0.36 | 0.87
Fourth quarterly distribution declared per unit (in dollars per unit) | $ 1.75 | $ 0.46 | $ 0.70

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