Principal Activity and Significant Accounting Policies - Principal Business Activity (Details)	Mar. 31, 2018 ft² property item
Properties
No. of properties	167
Residential Property
Properties
No. of properties	118
Percentage of residential property out of the trust properties	71.30%
Units | item	9,458
Residential Property | North Dakota
Properties
No. of properties	99
Units | item	5,951
Residential Property | Minnesota
Properties
No. of properties	16
Units | item	3,027
Residential Property | Missouri
Properties
No. of properties	1
Units | item	164
Residential Property | Nebraska
Properties
No. of properties	2
Units | item	316
Commercial Property
Properties
No. of properties	49
Percentage of commercial property out of the trust properties	28.70%
Area of commercial property | ft²	1,691,000
Commercial Property | North Dakota
Properties
No. of properties	20
Area of commercial property | ft²	805,000
Commercial Property | Arkansas
Properties
No. of properties	2
Area of commercial property | ft²	29,000
Commercial Property | Colorado
Properties
No. of properties	1
Area of commercial property | ft²	13,000
Commercial Property | Iowa
Properties
No. of properties	1
Area of commercial property | ft²	33,000
Commercial Property | Louisiana
Properties
No. of properties	1
Area of commercial property | ft²	15,000
Commercial Property | Michigan
Properties
No. of properties	1
Area of commercial property | ft²	12,000
Commercial Property | Minnesota
Properties
No. of properties	15
Area of commercial property | ft²	683,000
Commercial Property | Mississippi
Properties
No. of properties	1
Area of commercial property | ft²	15,000
Commercial Property | Nebraska
Properties
No. of properties	1
Area of commercial property | ft²	16,000
Commercial Property | Texas
Properties
No. of properties	1
Area of commercial property | ft²	7,000
Commercial Property | Wisconsin
Properties
No. of properties	5
Area of commercial property | ft²	63,000