Principal Activity and Significant Accounting Policies - Principal Business Activity (Details) $ in Thousands	Sep. 30, 2019 USD ($) ft² item property	Jun. 30, 2019 ft²	Dec. 31, 2018 USD ($)
Properties
Real estate investments | $	$ 654,747		$ 662,584
No. of properties	173
Area of commercial property | ft²		36,000
Residential Property
Properties
Percentage of residential property out of the trust properties	73.30%
No. of properties	125
Units | item	9,852
Residential Property | North Dakota
Properties
No. of properties	105
Units | item	6,160
Residential Property | Minnesota
Properties
No. of properties	16
Units | item	3,033
Residential Property | Missouri
Properties
No. of properties	1
Units | item	164
Residential Property | Nebraska
Properties
No. of properties	3
Units | item	495
Commercial Property
Properties
Percentage of commercial property out of the trust properties	26.70%
No. of properties	48
Area of commercial property | ft²	1,662,000
Commercial Property | North Dakota
Properties
No. of properties	20
Area of commercial property | ft²	780,000
Commercial Property | Minnesota
Properties
No. of properties	15
Area of commercial property | ft²	680,000
Commercial Property | Nebraska
Properties
No. of properties	1
Area of commercial property | ft²	19,000
Commercial Property | Arkansas
Properties
No. of properties	2
Area of commercial property | ft²	28,000
Commercial Property | Colorado
Properties
No. of properties	1
Area of commercial property | ft²	17,000
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 | Mississippi
Properties
No. of properties	1
Area of commercial property | ft²	15,000
Commercial Property | Wisconsin
Properties
No. of properties	5
Area of commercial property | ft²	63,000