Mortgages Payable (Details)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) item	Sep. 30, 2017 USD ($) property loan item	Sep. 30, 2016 USD ($) item	Dec. 31, 2016 USD ($) property loan item
Debt
Proceeds from mortgages payable			$ 9,963,000
Mortgage Payable Balance		$ 341,015,000		$ 466,045,000
Mortgages Payable
Debt
Amount repaid		123,500,000	183,700,000
Assumed mortgages payable		0	32,500,000
Number of assumed mortgages refinanced | item	1
Proceeds from mortgages payable	$ 10,000,000
Net premiums recorded upon acquisition of mortgages			692,000
Remaining balance of deferred financing costs at period end		$ 249,000		$ 324,000
Number of Properties | item		63		127
Weighted Average Stated Interest Rate (as a percent)		4.90%		4.90%
Weighted Average Effective Interest Rate (as a percent)		4.50%		4.30%
Weighted Average Remaining Years Until Maturity		4 years 3 months 18 days		4 years
Remaining Principal Balance		$ 336,484,000		$ 460,008,000
Unamortized Premium and Deferred Finance Costs Balance, net		4,531,000		6,037,000
Mortgage Payable Balance		$ 341,015,000		$ 466,045,000
Number of mortgages | loan		29		36
Number of properties with mortgages | property		63		127
Unamortized premium balance		$ 4,800,000
Maturity of mortgages payable, excluding net premiums
2017		1,300,000
2018		21,900,000
2019		20,700,000
2020		82,400,000
2021		66,900,000
Thereafter		143,300,000
Totals		$ 336,500,000
Mortgages Payable | Minimum
Debt
Weighted Average Stated Interest Rate (as a percent)		3.20%		2.40%
Weighted Average Effective Interest Rate (as a percent)		3.00%		2.50%
Mortgages Payable | Maximum
Debt
Weighted Average Stated Interest Rate (as a percent)		6.90%		6.90%
Weighted Average Effective Interest Rate (as a percent)		5.50%		8.80%
Mortgages Payable | Mortgages repaid in full
Debt
Amount repaid		$ 118,600,000	$ 161,500,000
Number of mortgages repaid | item		7	8
Mortgages Payable | Variable rate mortgages
Debt
Remaining Principal Balance		$ 44,900,000		$ 76,300,000
Number of mortgages | loan		4		6
Number of properties with mortgages | property		4		15
Mortgages Payable | Variable rate mortgages after interest rate arrangements
Debt
Remaining Principal Balance		$ 22,500,000		$ 38,200,000