Mortgages Payable (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item
Debt instrument
Remaining balance of deferred financing costs at period end	$ 20,490,000	$ 23,274,000
Mortgage Payable Balance	646,740,000	$ 852,575,000
Maturity mortgages payable, excluding net premiums
2016	275,000,000
2017	175,000,000
2018	350,000,000
2019	550,000,000
Thereafter	2,300,000,000
Totals	$ 3,650,000,000
Minimum
Debt instrument
Weighted Average Stated Interest Rate (as a percent)	2.00%	2.00%
Weighted Average Effective Interest Rate (as a percent)	2.20%	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)	8.90%	9.00%
Mortgages Payable
Debt instrument
Amount repaid	$ 198,400,000	$ 85,200,000
Assumed mortgages payable		166,700,000
Net premiums recorded upon acquisition of mortgages		(604,000)
Remaining balance of deferred financing costs at period end	$ 553,000	$ 827,000
Number of Properties | item	183	241
Weighted Average Stated Interest Rate (as a percent)	4.90%	5.00%
Weighted Average Effective Interest Rate (as a percent)	4.10%	4.00%
Weighted Average Remaining Years Until Maturity	3 years 7 months 6 days	3 years 8 months 12 days
Principal Balance	$ 637,658,000	$ 836,011,000
Unamortized Premium Balance	(9,082,000)	(16,564,000)
Mortgage Payable Balance	$ 646,740,000	$ 852,575,000
Number of mortgages | item	44	57
Number of properties with mortgages | item	183	241
Maturity mortgages payable, excluding net premiums
2016	$ 170,100,000
2017	142,900,000
2018	15,500,000
2019	26,300,000
2020	82,400,000
Thereafter	200,500,000
Totals	637,700,000
Mortgages repaid in full
Debt instrument
Amount repaid	$ 191,000,000	$ 77,800,000
Number of mortgages repaid | item	13	6
Variable rate mortgages
Debt instrument
Mortgage Payable Balance	$ 51,100,000	$ 74,500,000
Number of mortgages | item	4	5
Number of properties with mortgages | item	13	14
Variable Rate Mortgages After Interest Rate Arrangements
Debt instrument
Principal Balance	$ 15,500,000	$ 39,100,000
Number of mortgages | item	2	3