Name:	msb_CashAndCashEquivalentsAndUSGovernmentSecurities
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Represents the cash distributions per unit declared and paid by the entity during the period for all units of stock.

No definition available.

Name:	msb_DistributionDeclaredDistributionsPaidPerUnit
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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

The aggregate amount of distributions paid and declared in the period.

No definition available.

Name:	msb_DistributionsPaidAndDeclared
Namespace Prefix:	msb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the maintained level of unallocated reserve in the form of cash and U.S. Government securities.

No definition available.

Name:	msb_UnallocatedCashAndCashEquivalentsAndUSGovernmentSecurities
Namespace Prefix:	msb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The cumulative amount of the reporting entity's undistributed earnings or deficit, consisting primarily of accrued income receivable representing royalties not yet received by the Trust but anticipated to be received in future periods.

No definition available.

Name:	msb_UnallocatedReserve
Namespace Prefix:	msb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

Name:	msb_UnallocatedReserveAndDistributionsRollForward
Namespace Prefix:	msb_
Data Type:	xbrli:stringItemType
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n/a

No definition available.

Name:	msb_UnallocatedReserveComponentsAbstract
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Data Type:	xbrli:stringItemType
Balance Type:	na
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Sum of the carrying values as of the balance sheet date of (a) interest payable on all forms of debt, including trade payables, that has been incurred, and (b) dividends declared but unpaid on equity securities issued by the entity and outstanding (also includes dividends collected on behalf of another owner of securities that are being held by the entity). Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Period Type:	instant

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Net Income
-URI http://asc.fasb.org/extlink&oid=51831255

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(18))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Other Comprehensive Income
-URI http://asc.fasb.org/extlink&oid=51831270

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(20))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Cash outflow in the form of capital distributions and dividends to common shareholders, preferred shareholders and noncontrolling interests.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3291-108585