Mortgages Payable (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) loan item	Dec. 31, 2015 USD ($) item
Debt instrument
Assumed mortgages payable	$ 44,100,000	$ 0
Number of assumed mortgages refinanced | loan	1
Proceeds from mortgages payable	$ 9,963,000
Number of assumed mortgages with an above-market interest rate | loan	1
Mortgage Payable Balance	$ 466,045,000	$ 646,187,000
Maturity mortgages payable, excluding net premiums
2017	175,000,000
2018	350,000,000
2019	550,000,000
2021	250,000,000
Thereafter	2,650,000,000
Totals	$ 3,975,000,000
Minimum
Debt instrument
Weighted Average Stated Interest Rate (as a percent)	2.40%	2.00%
Weighted Average Effective Interest Rate (as a percent)	2.50%	2.20%
Maximum
Debt instrument
Weighted Average Stated Interest Rate (as a percent)	6.90%	6.90%
Weighted Average Effective Interest Rate (as a percent)	8.80%	8.90%
Mortgages Payable
Debt instrument
Amount repaid	$ 231,700,000	$ 198,400,000
Net premiums recorded upon acquisition of mortgages	692,000
Remaining balance of deferred financing costs at period end	$ 324,000	$ 553,000
Number of Properties | item	127	183
Weighted Average Stated Interest Rate (as a percent)	4.90%	4.90%
Weighted Average Effective Interest Rate (as a percent)	4.30%	4.10%
Weighted Average Remaining Years Until Maturity	4 years	3 years 7 months 6 days
Principal Balance	$ 460,008,000	$ 637,658,000
Unamortized Premium and Deferred Finance Costs Balance, net	6,037,000	8,529,000
Mortgage Payable Balance	$ 466,045,000	$ 646,187,000
Number of mortgages | item	36	44
Number of properties with mortgages | item	127	183
Maturity mortgages payable, excluding net premiums
2017	$ 103,200,000
2018	21,900,000
2019	42,300,000
2020	82,400,000
2021	66,900,000
Thereafter	143,300,000
Totals	460,000,000
Mortgages repaid in full
Debt instrument
Amount repaid	$ 201,800,000	$ 191,000,000
Number of mortgages repaid | item	11	13
Variable rate mortgages
Debt instrument
Mortgage Payable Balance	$ 74,000,000	$ 51,100,000
Number of mortgages | item	6	4
Number of properties with mortgages | item	15	13
Variable rate mortgages after interest rate arrangements
Debt instrument
Principal Balance	$ 38,200,000	$ 15,500,000
Number of mortgages | item	4	2