Mortgages Payable (Details)	6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) property loan item	Jun. 30, 2016 USD ($) item	Dec. 31, 2016 USD ($) property loan item
Debt
Mortgage Payable Balance	$ 378,352,000		$ 466,045,000
Minimum
Debt
Weighted Average Stated Interest Rate (as a percent)	3.00%		2.40%
Weighted Average Effective Interest Rate (as a percent)	3.00%		2.50%
Maximum
Debt
Weighted Average Stated Interest Rate (as a percent)	6.90%		6.90%
Weighted Average Effective Interest Rate (as a percent)	8.70%		8.80%
Variable rate mortgages
Debt
Remaining Principal Balance	$ 45,100,000.00000
Number of mortgages | loan	4
Number of properties with mortgages | property	4
Mortgages Payable
Debt
Amount repaid	$ 86,500,000	$ 181,300,000
Assumed mortgages payable	0	32,500,000
Net premiums recorded upon acquisition of mortgages		692,000
Remaining balance of deferred financing costs at period end	$ 262,000		$ 324,000
Number of Properties | item	72		127
Weighted Average Stated Interest Rate (as a percent)	5.10%		4.90%
Weighted Average Effective Interest Rate (as a percent)	4.50%		4.30%
Weighted Average Remaining Years Until Maturity	4 years 1 month 6 days		4 years
Remaining Principal Balance	$ 373,493,000		$ 460,008,000
Unamortized Premium and Deferred Finance Costs Balance, net	4,859,000		6,037,000
Mortgage Payable Balance	$ 378,352,000		$ 466,045,000
Number of mortgages | loan	31		36
Number of properties with mortgages | property	72		127
Unamortized premium balance	$ 5,100,000
Maturity of mortgages payable, excluding net premiums
2017	38,300,000
2018	21,900,000
2019	20,700,000
2020	82,400,000
2021	66,900,000
Thereafter	143,300,000
Totals	373,500,000
Mortgages Payable | Mortgages repaid in full
Debt
Amount repaid	$ 82,900,000	$ 161,500,000
Number of mortgages repaid | item	5	8
Mortgages Payable | Variable rate mortgages
Debt
Remaining Principal Balance			$ 76,300,000
Number of mortgages | loan			6
Number of properties with mortgages | property			15
Mortgages Payable | Variable rate mortgages after interest rate arrangements
Debt
Remaining Principal Balance	$ 22,600,000		$ 38,200,000