Represents the adjusted threshold price per ton as per the collaborative agreement.

No definition available.

Name:	msb_CollaborativeArrangementAdjustedThresholdPricePerTon
Namespace Prefix:	msb_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the adjusted threshold price per ton, subject to adjustment for inflation and deflation but not below a specified limit, as per the collaborative agreement.

+ References

No definition available.

+ Details

Name:	msb_CollaborativeArrangementAdjustedThresholdPricePerTonSubjectToAdjustment
Namespace Prefix:	msb_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the advance royalty to be received by the entity on a quarterly basis under the assignment agreement. This amount consists of an annual amount adjusted for inflation and deflation.

+ References

No definition available.

+ Details

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

- Definition

Represents the advance royalty to be received by the entity on a quarterly basis, during the next calendar year, under the assignment agreement. This amount consists of an annual amount adjusted for inflation and deflation.

+ References

No definition available.

+ Details

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

- Definition

Represents the period within which the related party is obligated to make payments of overriding royalties on product shipments, following the calendar quarter in which such shipments occur, as per the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=msb_CyprusNorthshoreMiningCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

ROYALTY AGREEMENT (Details) - Cyprus NMC	12 Months Ended
ROYALTY AGREEMENT (Details) - Cyprus NMC	Dec. 31, 2018 USD ($) $ / T	Jan. 31, 2018 USD ($) $ / T	Dec. 31, 2017 USD ($) $ / T	Dec. 31, 2016 USD ($) $ / T
ROYALTY AGREEMENT
Adjusted threshold price (in dollars per ton)			54.74	53.80
Adjusted threshold price subject to adjustment (in dollars per ton)	55.74
Maximum
ROYALTY AGREEMENT
Period for payments of overriding royalties on product shipments		30 days
Minimum
ROYALTY AGREEMENT
Adjusted threshold price (in dollars per ton)		30
Advance royalty threshold \| $		$ 500,000	$ 912,725	$ 897,008
Advance royalty threshold, next calendar year \| $	$ 929,320