Investments in Real Estate (Details) (USD $)	3 Months Ended
	Mar. 31, 2012 sqft	Mar. 31, 2011 sqft
New properties acquired [Abstract]
Investments in new properties	$ 10,700,000	$ 150,700,000
Number of new properties	2	26
New properties weighted average contractual lease rate (in hundredths)	9.00%	7.90%
New properties U.S. states of location	2	15
New properties leasable square feet (in square feet)	34,000	1,300,000
New properties leased percentage (in hundredths)	100.00%	100.00%
New properties average lease term (in years)	15	16.6
Acquisition transaction costs	242,000	371,000
Capitalized costs on existing properties	1,100,000	943,000
Re-leasing costs included in capitalized costs on existing properties	266,000	269,000
Building and tenant improvements included in capitalized costs on existing properties	793,000	674,000
Intangible value of the in-place leases acquired		21,800,000
Intangible value of above-market leases acquired		11,100,000
Intangible value of below-market leases acquired		833,000
Amortization of above and below Market Leases	483,000	46,000
Value of in place leases amortized to expense	3,300,000	629,000
Real Estate Investments With Existing Leases [Member]
New properties acquired [Abstract]
Investments in new properties		$ 130,100,000
Number of new properties		13