Mortgages Payable (Details) (USD $)	1 Months Ended	9 Months Ended	12 Months Ended
	Aug. 31, 2013 item	Sep. 30, 2013 item	Dec. 31, 2012 item
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
Mortgage Payable Balance		811,058,000	175,868,000
Minimum
Debt instrument
Weighted Average Stated Interest Rate (as a percent)		2.50%	2.60%
Weighted Average Effective Interest Rate (as a percent)		2.40%	2.70%
Maximum
Debt instrument
Weighted Average Stated Interest Rate (as a percent)		8.30%	8.30%
Weighted Average Effective Interest Rate (as a percent)		9.20%	8.30%
Mortgages Payable
Debt instrument
Number of mortgages repaid	1
Amount repaid	11,700,000
Net premiums recorded upon assumption of mortgages		(28,400,000)	(7,100,000)
Deferred financing costs incurred		211,000	1,100,000
Remaining balance of deferred financing costs at period end		1,346,000	1,541,000
Number of Properties		229	11
Weighted Average Stated Interest Rate (as a percent)		5.40%	5.80%
Weighted Average Effective Interest Rate (as a percent)		4.00%	4.40%
Weighted Average Remaining Years Until Maturity		4 years 1 month 6 days	4 years 9 months 18 days
Remaining Principal Balance		779,684,000	165,927,000
Unamortized Premium Balance		31,373,000	9,941,000
Mortgage Payable Balance		811,058,000	175,868,000
Number of mortgages		50	13
Number of properties with mortgages		229	11
Maturity mortgages payable, excluding net premiums
2013		10,700,000
2014		64,400,000
2015		125,500,000
2016		248,400,000
2017		133,100,000
Thereafter		197,600,000
Totals		779,700,000
Mortgages maturing 9/3/21
Debt instrument
Remaining Principal Balance		8,300,000
Floating variable interest rate, variable rate basis		one month LIBOR
Floating variable interest rate, basis spread on variable rate (as a percent)		2.40%
Assumed interest rate swap fixed interest rate (as a percent)		6.00%
Mortgages maturing 6/10/15
Debt instrument
Remaining Principal Balance		23,600,000
Floating variable interest rate, variable rate basis		one month LIBOR	one month LIBOR
Floating variable interest rate, basis spread on variable rate (as a percent)		4.50%	4.50%
Maximum interest rate (as a percent)		5.50%	5.50%
Mortgages maturing 4/10/17
Debt instrument
Weighted Average Stated Interest Rate (as a percent)		5.07%
Remaining Principal Balance		32,400,000
Floating variable interest rate, variable rate basis		two-year swap rate
Floating variable interest rate, basis spread on variable rate (as a percent)		2.75%
Minimum interest rate (as a percent)		4.00%
Mortgages maturing 12/28/13
Debt instrument
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%