Mortgages Payable (Details) (USD $)	6 Months Ended						6 Months Ended	12 Months Ended	6 Months Ended		12 Months Ended	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Minimum	Dec. 31, 2012 Minimum	Jun. 30, 2013 Maximum	Dec. 31, 2012 Maximum	Jun. 30, 2013 Mortgages Payable item	Dec. 31, 2012 Mortgages Payable item	Jun. 30, 2012 Mortgages maturing 9/3/21	Jun. 30, 2013 Mortgages maturing 6/10/15	Dec. 31, 2012 Mortgages maturing 6/10/15	Jun. 30, 2013 Mortgages maturing 4/10/17	Jun. 30, 2013 Mortgages maturing 12/28/13	Dec. 31, 2012 Mortgages maturing 12/28/13
Mortgages Payable
Assumed mortgages payable	$ 630,000,000
Assumed mortgages payable, non-recourse	608,800,000
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							28,400,000	10,000,000
Deferred financing costs incurred							211,000	1,100,000
Remaining balance of deferred financing costs at period end							1,488,000	1,541,000
Number of Properties							229	11
Weighted Average Stated Interest Rate (as a percent)			2.50%	2.60%	8.30%	8.30%	5.40%	5.80%				5.07%
Weighted Average Effective Interest Rate (as a percent)			2.40%	2.70%	9.30%	8.30%	3.90%	4.40%
Weighted Average Remaining Years Until Maturity							4 years 6 months	4 years 9 months 18 days
Remaining Principal Balance							793,217,000	165,927,000	8,300,000	23,600,000		32,400,000
Unamortized Premium Balance							33,892,000	9,941,000
Mortgage Payable Balance	827,109,000	175,868,000					827,109,000	175,868,000
Number of mortgages							51	13
Number of properties with mortgages							229	11
Floating variable interest rate, variable rate basis									one month LIBOR	one month LIBOR	one month LIBOR	two-year swap rate
Floating variable interest rate, basis spread on variable rate (as a percent)									2.40%	4.50%	4.50%	2.75%
Maximum interest rate (as a percent)										5.50%	5.50%	4.00%
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%
Assumed interest rate swap fixed interest rate (as a percent)									6.00%
Maturity mortgages payable, excluding net premiums
2013							24,300,000
2014							64,400,000
2015							125,500,000
2016							248,400,000
2017							133,000,000
Thereafter							197,600,000
Totals							$ 793,200,000