Mortgages Payable (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2014 item	Dec. 31, 2013 item	Mar. 31, 2013
Mortgages Payable
Assumed mortgages payable	$ 45,900,000
Debt instrument
Unamortized Premium Balance	(23,700,000)
Mortgage Payable Balance	822,270,000	783,360,000
Maturity mortgages payable, excluding net premiums
2015	150,000,000
2016	275,000,000
2017	175,000,000
2018	350,000,000
Thereafter	2,250,000,000
Totals	3,200,000,000
Minimum
Debt instrument
Weighted Average Stated Interest Rate (as a percent)	2.00%	2.50%
Weighted Average Effective Interest Rate (as a percent)	2.20%	2.40%
Maximum
Debt instrument
Weighted Average Stated Interest Rate (as a percent)	6.90%	6.90%
Weighted Average Effective Interest Rate (as a percent)	9.10%	9.20%
Mortgages Payable
Debt instrument
Net discount (premiums) recorded upon assumption of mortgages	2,800,000	(28,400,000)	(26,800,000)
Remaining balance of deferred financing costs at period end	1,118,000	1,219,000
Number of Properties	239	227
Weighted Average Stated Interest Rate (as a percent)	5.20%	5.30%
Weighted Average Effective Interest Rate (as a percent)	3.80%	3.90%
Weighted Average Remaining Years Until Maturity	3 years 10 months 24 days	4 years 3 months 18 days
Principal Balance	798,580,000	754,508,000
Unamortized Premium Balance	23,690,000	28,852,000
Mortgage Payable Balance	822,270,000	783,360,000
Number of mortgages	50	47
Number of properties with mortgages	239	227
Maturity mortgages payable, excluding net premiums
2014	48,100,000
2015	125,500,000
2016	248,500,000
2017	142,400,000
2018	15,000,000
Thereafter	219,100,000
Totals	798,600,000
Variable rate mortgages
Debt instrument
Principal Balance	$ 105,100,000
Number of mortgages	6
Number of properties with mortgages	56