Name:	msb_FixedPropertyIncludingIntangiblesAbstract
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Represents the percentage of fee interest held by Mesabi Land Trust in the lands subject to the Peters Lease.

No definition available.

Name:	msb_PercentageOfFeeInterestHeldByLandTrustInLeaseProperty
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Data Type:	num:percentItemType
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The ceiling quantity for the first tier of base overriding royalties earned. Shipments up to this amount are subject to the first tier portion percentage in calculating the royalties earned.

No definition available.

Name:	msb_RevenueCeilingQuantityForFirstTierOfConsideration
Namespace Prefix:	msb_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	duration

The ceiling quantity for the second tier of base overriding royalties earned. Shipments up to this amount, above the first tier ceiling, are subject to the second tier portion percentage in calculating the royalties earned.

No definition available.

Name:	msb_RevenueCeilingQuantityForSecondTierOfConsideration
Namespace Prefix:	msb_
Data Type:	num:massItemType
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The number of volume thresholds used to determine the transaction price.

No definition available.

Name:	msb_RevenueNumberOfVolumeThresholdsForVariableTransactionPrice
Namespace Prefix:	msb_
Data Type:	xbrli:integerItemType
Balance Type:	na
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The percentage applied to the quantity shipped in the first tier, to determine the quantity on which royalties are earned.

No definition available.

Name:	msb_RevenuePortionPercentageForFirstTierOfConsideration
Namespace Prefix:	msb_
Data Type:	num:percentItemType
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The percentage applied to the quantity shipped in the second tier, to determine the quantity on which royalties are earned.

No definition available.

Name:	msb_RevenuePortionPercentageForSecondTierOfConsideration
Namespace Prefix:	msb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The percentage applied to the quantity shipped in the third tier, to determine the quantity on which royalties are earned.

No definition available.

Name:	msb_RevenuePortionPercentageForThirdTierOfConsideration
Namespace Prefix:	msb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The number of revenue-generating contracts giving rise to contract asset and liability.

No definition available.

Name:	msb_RevenueRecognitionNumberOfContracts
Namespace Prefix:	msb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

The threshold quantity for the third tier of base overriding royalties earned. Shipments above this amount are subject to the third tier portion percentage in calculating the royalties earned.

No definition available.

Name:	msb_RevenueThresholdQuantityForThirdTierOfConsideration
Namespace Prefix:	msb_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	duration

The quantity shipped which serves as a threshold to determine transaction price.

No definition available.

Name:	msb_RevenueVolumeThresholdForTransactionPrice
Namespace Prefix:	msb_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	us-gaap_CashAndCashEquivalentsAbstract
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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130533-203044

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

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

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130532-203044

Name:	us-gaap_ContractWithCustomerLiability
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Data Type:	xbrli:monetaryItemType
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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

No definition available.

Name:	us-gaap_MoneyMarketFundsAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.