Real Estate Investments (Summary of Intangible Assets Acquired and Intangible Liabilities Assumed) (Details) (Three Logan Square [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Years	Aug. 05, 2010
Intangible assets:
Intangible assets acquired		$ 28,856
Weighted Average Amortization Period (in years)	23
Intangible liabilities:
Below market leases assumed		683
Leases, Acquired-in-Place [Member]
Intangible assets:
Intangible assets acquired		13,584
Weighted Average Amortization Period (in years)	3
Customer Relationships [Member]
Intangible assets:
Intangible assets acquired		8,870
Weighted Average Amortization Period (in years)	5
Above market tenant leases acquired [Member]
Intangible assets:
Intangible assets acquired		895
Weighted Average Amortization Period (in years)	1
Below market ground lease acquired [Member]
Intangible assets:
Intangible assets acquired		5,507
Weighted Average Amortization Period (in years)	82
Intangible liabilities:
Below market leases assumed		$ 683
Weighted Average Amortization Period (in years)	1