Summary of Significant Accounting Policies (Narrative) (Details) - ft² ft² in Millions	1 Months Ended	9 Months Ended
	Feb. 28, 2017	Sep. 30, 2017
Significant Accounting Policies [Line Items]
Square footage of operating properties (in square feet)		42.4
Deferred compensation plan, period to diversify common shares after vesting	6 months
Base Minimum Rental Revenue [Member] | Tenant Base [Member]
Significant Accounting Policies [Line Items]
Concentrations of risk (percentage)		2.90%
Base Minimum Rental Revenue [Member] | Houston, Texas Geographic Concentration [Member]
Significant Accounting Policies [Line Items]
Concentrations of risk (percentage)		20.20%
Base Minimum Rental Revenue [Member] | Other Parts of Texas Geographic Concentration [Member]
Significant Accounting Policies [Line Items]
Concentrations of risk (percentage)		9.20%