Investments in Unconsolidated Joint Ventures - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 sqft acre	Dec. 31, 2010	Dec. 31, 2009
Central Region [Member] | Liberty Illinois, LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%
Square Footage of Real Estate Property	5,100,000
Acres of Developable Land	335
Management Fees Revenue	$ 635,000	$ 596,000	$ 890,000
United Kingdom [Member] | Kings Hill Unit Trust [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%
Square Footage of Real Estate Property	535,000
Management Fees Revenue	427,000	465,000	492,000
United Kingdom [Member] | Blythe Valley JV Sarl [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%
Square Footage of Real Estate Property	457,000
Acres of Developable Land	98
Management Fees Revenue	335,000	316,000	376,000
United Kingdom [Member] | Silversword Properties Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
United Kingdom [Member] | Cambridge Medipark Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Number of Real Estate Properties	0
South Region [Member] | Liberty AIPO, LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Northeast Region [Member] | Liberty Venture I, LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%
Square Footage of Real Estate Property	3,100,000
Acres of Developable Land	43
Management Fees Revenue	614,000	611,000	682,000
Metro [Member] | Liberty Washington, LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%
Square Footage of Real Estate Property	2,600,000
Acres of Developable Land	6
Management Fees Revenue	4,000,000	4,000,000	4,100,000
Metro [Member] | Liberty/Commerz 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%
Square Footage of Real Estate Property	1,250,000
Management Fees Revenue	$ 2,100,000	$ 2,000,000	$ 1,900,000
Industrial Properties [Member] | Central Region [Member] | Liberty Illinois, LP [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	15
Industrial Properties [Member] | United Kingdom [Member] | Kings Hill Unit Trust [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	5
Industrial Properties [Member] | South Region [Member] | Liberty AIPO, LP [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	4
Industrial Properties [Member] | Northeast Region [Member] | Liberty Venture I, LP [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	23
Office Buildings [Member] | United Kingdom [Member] | Kings Hill Unit Trust [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	10
Office Buildings [Member] | United Kingdom [Member] | Blythe Valley JV Sarl [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	12
Office Buildings [Member] | United Kingdom [Member] | Silversword Properties Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	1
Office Buildings [Member] | Metro [Member] | Liberty Washington, LP [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	25
Office Buildings [Member] | Metro [Member] | Liberty/Commerz 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties	1