Mortgages Payable (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015 item	Mar. 31, 2014	Dec. 31, 2014 item
Debt instrument
Assumed mortgages payable		$ 45,900,000
Mortgage Payable Balance	799,318,000		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)	9.00%		9.00%
Mortgages Payable
Debt instrument
Net discount (premiums) recorded upon assumption of mortgages		2,800,000
Deferred financing costs incurred	750,000		827,000
Number of Properties	238		241
Weighted Average Stated Interest Rate (as a percent)	5.00%		5.00%
Weighted Average Effective Interest Rate (as a percent)	4.00%		4.00%
Weighted Average Remaining Years Until Maturity	3 years 8 months 12 days		3 years 8 months 12 days
Principal Balance	784,715,000		836,011,000
Unamortized Premium Balance	14,603,000		16,564,000
Mortgage Payable Balance	799,318,000		852,575,000
Number of mortgages	54		57
Number of properties with mortgages	238		241
Maturity mortgages payable, excluding net premiums
2015	68,400,000
2016	248,400,000
2017	142,500,000
2018	15,100,000
2019	26,000,000
Thereafter	284,300,000
Totals	784,700,000
Amount repaid	51,300,000	1,800,000
Mortgages repaid in full
Maturity mortgages payable, excluding net premiums
Number of mortgages repaid	3
Amount repaid	49,400,000
Variable rate mortgages
Debt instrument
Principal Balance	$ 50,900,000		$ 74,500,000
Number of mortgages	4		5
Number of properties with mortgages	13		14