STATEMENTS OF UNALLOCATED RESERVE AND TRUST CORPUS (Parenthetical) (USD $)	3 Months Ended
Jan. 31, 2015	Oct. 31, 2014	Jul. 31, 2014	Jan. 31, 2014	Oct. 31, 2013	Jul. 31, 2013	Jan. 31, 2013	Oct. 31, 2012	Jul. 31, 2012
STATEMENTS OF UNALLOCATED RESERVE AND TRUST CORPUS
Distribution paid (in dollars per unit)	$ 0.88msb_DistributionPaidPerUnit	$ 0.32msb_DistributionPaidPerUnit	$ 0.00msb_DistributionPaidPerUnit	$ 0.51msb_DistributionPaidPerUnit	$ 0.46msb_DistributionPaidPerUnit	$ 0.08msb_DistributionPaidPerUnit	$ 1.13msb_DistributionPaidPerUnit	$ 0.64msb_DistributionPaidPerUnit	$ 0.065msb_DistributionPaidPerUnit
Distributions declared per unit (in dollars per unit)	$ 0.64msb_DistributionDeclaredPerUnit			$ 0.57msb_DistributionDeclaredPerUnit			$ 0.49msb_DistributionDeclaredPerUnit

Cash dividends declared per unit by an entity during the period for all units of stock and subsequently paid within the following period.

No definition available.

Name:	msb_DistributionDeclaredPerUnit
Namespace Prefix:	msb_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration