Mortgages Payable (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) property loan	Mar. 31, 2017 USD ($) loan	Dec. 31, 2017 USD ($) property loan
Debt
Mortgage Payable Balance	$ 313,173,000		$ 325,941,000
Mortgages Payable
Debt
Amount repaid	$ 12,300,000	$ 2,100,000
Number of assumed mortgages refinanced | loan	0	0
Remaining balance of deferred financing costs at period end	$ 223,000		$ 236,000
Number of Properties | property	61		62
Weighted Average Stated Interest Rate	5.00%		5.00%
Weighted Average Effective Interest Rate (as a percent)	4.50%		4.40%
Weighted Average Remaining Years Until Maturity	3 years 10 months 24 days		4 years
Remaining Principal Balance	$ 307,960,000		$ 320,283,000
Unamortized Premium and Deferred Finance Costs Balance, net	5,213,000		5,658,000
Mortgage Payable Balance	$ 313,173,000		$ 325,941,000
Number of mortgages | loan	27		28
Mortgages Payable | Minimum
Debt
Weighted Average Effective Interest Rate (as a percent)	2.30%		2.60%
Stated interest rate (as a percent)	3.70%		3.40%
Mortgages Payable | Maximum
Debt
Weighted Average Effective Interest Rate (as a percent)	5.70%		5.50%
Stated interest rate (as a percent)	6.90%		6.90%
Mortgages Payable | Mortgages repaid in full
Debt
Amount repaid	$ 11,000,000
Number of mortgages repaid | loan	1
Mortgages Payable | Variable rate mortgages
Debt
Remaining Principal Balance	$ 29,700,000		$ 29,900,000
Number of mortgages | loan	3		3
Number of properties with mortgages | property	3		3
Mortgages Payable | Variable rate mortgages after interest rate arrangements
Debt
Remaining Principal Balance	$ 22,300,000		$ 22,400,000