Investments in Unconsolidated Joint Ventures - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 acre sqft		Dec. 31, 2011		Dec. 31, 2010
Schedule of Equity Method Investments [Line Items]
Company's net investment in unconsolidated joint ventures	$ 169,021,000		$ 174,687,000
Liberty Venture I, LP [Member]
Schedule of Equity Method Investments [Line Items]
Company's net investment in unconsolidated joint ventures	8,205,000	[1]	8,428,000	[1]
Kings Hill Unit Trust [Member]
Schedule of Equity Method Investments [Line Items]
Company's net investment in unconsolidated joint ventures	10,341,000	[1]	9,634,000	[1]
Liberty Illinois, LP [Member]
Schedule of Equity Method Investments [Line Items]
Company's net investment in unconsolidated joint ventures	21,331,000	[1]	21,348,000	[1]
Blythe Valley JV Sarl [Member]
Schedule of Equity Method Investments [Line Items]
Company's net investment in unconsolidated joint ventures	0	[1]	3,663,000	[1]
Liberty Washington, LP [Member]
Schedule of Equity Method Investments [Line Items]
Company's net investment in unconsolidated joint ventures	76,965,000	[1]	74,893,000	[1]
Liberty Commerz 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Company's net investment in unconsolidated joint ventures	27,305,000	[1]	31,615,000	[1]
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	655,000		635,000		596,000
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	532,000
Management Fees Revenue	333,000		427,000		465,000
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	355,000		335,000		316,000
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	619,000		614,000		611,000
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	4,500,000		4,000,000		4,000,000
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	$ 2,200,000		$ 2,100,000		$ 2,000,000
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

[1] Differences between the Company's net investment in unconsolidated joint ventures and its underlying equity in the net assets of the venture are primarily a result of impairments related to the Company's investment in unconsolidated joint ventures, the deferral of gains associated with the sales of properties to joint ventures in which the Company retains an ownership interest and loans made to the joint ventures by the Company. These adjustments have resulted in an aggregate difference reducing the Company's investments in unconsolidated joint ventures by $28.5 million and $42.7 million as of December 31, 2012 and 2011, respectively. Differences between historical cost basis and the basis reflected at the joint venture level (other than loans) are typically depreciated over the life of the related asset.