Mortgages Payable (Details) (USD $)	3 Months Ended				3 Months Ended	12 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Minimum	Mar. 31, 2013 Maximum	Mar. 31, 2013 Mortgages Payable item	Dec. 31, 2012 Mortgages Payable item	Mar. 31, 2012 Mortgages maturing 9/3/21	Mar. 31, 2013 Mortgages maturing 6/10/15	Dec. 31, 2012 Mortgages maturing 6/10/15	Mar. 31, 2013 Mortgages maturing 12/28/13	Dec. 31, 2012 Mortgages maturing 12/28/13
Mortgages Payable
Assumed mortgages payable	$ 564,500,000
Assumed mortgages payable, non-recourse	543,300,000
Assumed mortgages payable, full recourse	6,600,000
Assumed mortgages payable, not guaranteed	14,600,000
Debt instrument
Net premiums recorded upon assumption of mortgages					26,800,000	10,000,000
Deferred financing costs incurred						1,100,000
Remaining balance of deferred financing costs at period end					1,414,000	1,541,000
Number of Properties					185	11
Weighted Average Stated Interest Rate (as a percent)			2.60%	8.30%	5.40%	5.80%
Weighted Average Effective Interest Rate (as a percent)			2.40%	9.30%	3.80%	4.40%
Weighted Average Remaining Years Until Maturity					4 years 4 months 24 days	4 years 9 months 18 days
Remaining Principal Balance					729,320,000	165,927,000	8,400,000	23,600,000
Unamortized Premium Balance					34,820,000	9,941,000
Mortgage Payable Balance	764,140,000	175,868,000			764,140,000	175,868,000
Number of mortgages					48	13
Number of properties with mortgages					185	11
Floating variable interest rate, variable rate basis							one month LIBOR	one month LIBOR	one month LIBOR
Floating variable interest rate, basis spread on variable rate (as a percent)							2.40%	4.50%	4.50%
Maximum interest rate (as a percent)								5.50%	5.50%
Note receivable acquired in connection with 2011 acquisition										8,800,000	8,800,000
Assumed note receivable stated interest rate (as a percent)										8.10%	8.10%
Assumed interest rate swap fixed interest rate (as a percent)							6.00%
Maturity mortgages payable, excluding net premiums
2013					24,800,000
2014					62,100,000
2015					123,200,000
2016					245,700,000
2017					90,000,000
Thereafter					183,500,000
Totals					$ 729,300,000