Mortgages Payable (Details) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2012 item	Sep. 30, 2011	Dec. 31, 2011
Mortgages Payable
Assumed mortgages payable	$ 70,000,000	$ 67,400,000
Debt instrument
Remaining Principal Balance	125,933,000		67,150,000
Amortized Premium (Discount) Balance	7,461,000		631,000
Mortgage Payable Balance	133,394,000		67,781,000
Note receivable acquired in connection with 2011 acquisition			8,800,000
Mortgages Payable
Debt instrument
Number of mortgages repaid	1
Amount repaid	10,700,000
Net premiums recorded upon assumption of mortgages	7,100,000		820,000
Deferred financing costs incurred	685,000	917,000
Remaining balance of deferred financing costs at period end	1,200,000		751,000
Mortgages maturing 5/6/12
Debt instrument
Stated Interest Rate (as a percent)			5.90%
Effective Interest Rate (as a percent)			5.20%
Remaining Principal Balance			10,664,000
Amortized Premium (Discount) Balance			26,000
Mortgage Payable Balance			10,690,000
Mortgages maturing 12/1/13
Debt instrument
Stated Interest Rate (as a percent)	6.30%		6.30%
Effective Interest Rate (as a percent)	4.60%		4.60%
Remaining Principal Balance	12,095,000		12,410,000
Amortized Premium (Discount) Balance	219,000		314,000
Mortgage Payable Balance	12,314,000		12,724,000
Number of properties occupied by applicable tenant	1		1
Mortgages maturing 9/1/14
Debt instrument
Stated Interest Rate (as a percent)	6.30%		6.30%
Effective Interest Rate (as a percent)	5.10%		5.10%
Remaining Principal Balance	11,550,000		11,671,000
Amortized Premium (Discount) Balance	226,000		359,000
Mortgage Payable Balance	11,776,000		12,030,000
Number of properties occupied by applicable tenant	1		1
Mortgages maturing 6/10/15
Debt instrument
Stated Interest Rate (as a percent)	4.70%		4.70%
Effective Interest Rate (as a percent)	4.80%		4.80%
Remaining Principal Balance	23,625,000		23,625,000
Amortized Premium (Discount) Balance	(53,000)		(68,000)
Mortgage Payable Balance	23,572,000		23,557,000
Floating variable interest rate, variable rate basis	1 month LIBOR		1 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 maturity 1/10/16
Debt instrument
Stated Interest Rate (as a percent)	6.00%
Effective Interest Rate (as a percent)	3.70%
Remaining Principal Balance	13,048,000
Amortized Premium (Discount) Balance	859,000
Mortgage Payable Balance	13,907,000
Mortgages maturing 6/6/17
Debt instrument
Stated Interest Rate (as a percent)	5.70%
Effective Interest Rate (as a percent)	2.70%
Remaining Principal Balance	10,150,000
Amortized Premium (Discount) Balance	1,269,000
Mortgage Payable Balance	11,419,000
Mortgages maturing 10/1/20
Debt instrument
Stated Interest Rate (as a percent)	6.00%
Effective Interest Rate (as a percent)	4.30%
Remaining Principal Balance	8,983,000
Amortized Premium (Discount) Balance	936,000
Mortgage Payable Balance	9,919,000
Mortgages maturing 9/3/21
Debt instrument
Stated Interest Rate (as a percent)	2.60%
Effective Interest Rate (as a percent)	4.00%
Remaining Principal Balance	8,394,000
Amortized Premium (Discount) Balance	(793,000)
Mortgage Payable Balance	7,601,000
Floating variable interest rate, variable rate basis	1 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 7/8/22
Debt instrument
Stated Interest Rate (as a percent)	6.40%
Effective Interest Rate (as a percent)	4.00%
Remaining Principal Balance	29,308,000
Amortized Premium (Discount) Balance	4,798,000
Mortgage Payable Balance	34,106,000
Mortgages one maturing 12/28/13
Debt instrument
Stated Interest Rate (as a percent)	8.30%		8.30%
Effective Interest Rate (as a percent)	8.30%		8.30%
Remaining Principal Balance	4,510,000		4,510,000
Mortgage Payable Balance	4,510,000		4,510,000
Number of properties occupied by applicable tenant	1		1
Mortgages two maturing 12/28/13
Debt instrument
Stated Interest Rate (as a percent)	8.30%		8.30%
Effective Interest Rate (as a percent)	8.30%		8.30%
Remaining Principal Balance	4,270,000		4,270,000
Mortgage Payable Balance	4,270,000		4,270,000
Number of properties occupied by applicable tenant	1		1
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)	8.14%		8.14%