Mortgages Payable (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($) item	Dec. 31, 2014 USD ($) item
Debt instrument
Assumed mortgages payable		$ 166,700,000
Net premiums recorded upon acquisition of mortgages		604,000
Mortgage Payable Balance	$ 706,141,000		$ 852,575,000
Maturity mortgages payable, excluding net premiums
2015	150,000,000
2016	275,000,000
2017	175,000,000
2018	350,000,000
2019	550,000,000
Thereafter	2,300,000,000
Totals	$ 3,800,000,000
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
Net premiums recorded upon acquisition of mortgages		604,000
Deferred financing costs incurred	$ 619,000		$ 827,000
Number of Properties | item	192		241
Weighted Average Stated Interest Rate (as a percent)	4.90%		5.00%
Weighted Average Effective Interest Rate (as a percent)	4.00%		4.00%
Weighted Average Remaining Years Until Maturity	3 years 7 months 6 days		3 years 8 months 12 days
Principal Balance	$ 695,186,000		$ 836,011,000
Unamortized Premium Balance	10,955,000		16,564,000
Mortgage Payable Balance	$ 706,141,000		$ 852,575,000
Number of mortgages | item	47		57
Number of properties with mortgages | item	192		241
Maturity mortgages payable, excluding net premiums
2015	$ 1,900,000
2016	225,600,000
2017	142,700,000
2018	15,300,000
2019	26,200,000
Thereafter	283,500,000
Totals	695,200,000
Amount repaid	$ 140,800,000	$ 77,600,000
Mortgages repaid in full
Maturity mortgages payable, excluding net premiums
Number of mortgages repaid | item	10	5
Amount repaid	$ 135,300,000	$ 72,000,000
Variable rate mortgages
Debt instrument
Principal Balance	$ 51,000,000		$ 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