Investments in Unconsolidated Joint Ventures - Narrative (Details) (USD $)																												12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Liberty Venture I, LP [Member]		Dec. 31, 2012 Liberty Venture I, LP [Member]		Dec. 31, 2013 Kings Hill Unit Trust [Member]		Dec. 31, 2012 Kings Hill Unit Trust [Member]		Dec. 31, 2013 Liberty Illinois, LP [Member]		Dec. 31, 2012 Liberty Illinois, LP [Member]		Dec. 31, 2012 Blythe Valley JV Sarl [Member]		Dec. 31, 2013 Liberty Washington, LP [Member]		Dec. 31, 2012 Liberty Washington, LP [Member]		Dec. 31, 2013 Liberty Commerz 1701 JFK Boulevard, LP [Member]		Dec. 31, 2012 Liberty Commerz 1701 JFK Boulevard, LP [Member]		Dec. 31, 2013 Industrial Other [Member] Liberty Venture I, LP [Member] acre sqft	Dec. 31, 2013 Industrial Other [Member] Liberty AIPO, LP [Member]	Dec. 31, 2013 United Kingdom [Member] Kings Hill Unit Trust [Member] sqft	Dec. 31, 2012 United Kingdom [Member] Blythe Valley JV Sarl [Member]	Dec. 31, 2013 United Kingdom [Member] Silversword Properties Ltd [Member]	Dec. 31, 2013 United Kingdom [Member] Cambridge Medipark Ltd [Member] properties	Dec. 31, 2013 Chicago Milwaukee [Member] Liberty Illinois, LP [Member] sqft acre	Dec. 31, 2013 Other Segments [Member] Liberty Washington, LP [Member] acre sqft	Dec. 31, 2013 Philadelphia [Member] Liberty Commerz 1701 JFK Boulevard, LP [Member] sqft	Dec. 31, 2013 Philadelphia [Member] Liberty Property 19th & Arch LP [Member] acre	Dec. 31, 2013 Construction in Progress [Member] properties sqft	Dec. 31, 2013 Construction in Progress [Member] Industrial Other [Member] Liberty Venture I, LP [Member] sqft	Dec. 31, 2013 Industrial Property [Member] Industrial Other [Member] Liberty Venture I, LP [Member] properties	Dec. 31, 2013 Industrial Property [Member] Industrial Other [Member] Liberty AIPO, LP [Member] properties	Dec. 31, 2013 Industrial Property [Member] United Kingdom [Member] Kings Hill Unit Trust [Member] properties	Dec. 31, 2013 Industrial Property [Member] Chicago Milwaukee [Member] Liberty Illinois, LP [Member] properties	Dec. 31, 2013 Industrial Property [Member] Construction in Progress [Member] Industrial Other [Member] Liberty Venture I, LP [Member] properties	Dec. 31, 2013 Office Building [Member] United Kingdom [Member] Kings Hill Unit Trust [Member] properties	Dec. 31, 2013 Office Building [Member] United Kingdom [Member] Silversword Properties Ltd [Member] properties	Dec. 31, 2013 Office Building [Member] Other Segments [Member] Liberty Washington, LP [Member] properties	Dec. 31, 2013 Office Building [Member] Philadelphia [Member] Liberty Commerz 1701 JFK Boulevard, LP [Member] properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage																									25.00%	50.00%	20.00%	20.00%	50.00%	50.00%	25.00%	25.00%	20.00%	50.00%
Number of Real Estate Properties																														0					16		23	4	3	15	1	11	1	21	1
Square Footage of Real Estate Property																									3,100,000		490,000				5,100,000	2,400,000	1,250,000		5,000,000	203,000
Anticipated aggregate investment																																			$ 380,800,000	$ 11,800,000
Acres of Developable Land																									43						335	6		1
Company's net investment in unconsolidated joint ventures	179,655,000	169,021,000	8,551,000	[1]	8,205,000	[1]	12,772,000	[1]	10,341,000	[1]	21,688,000	[1]	21,331,000	[1]	0	[1]	72,981,000	[1]	76,965,000	[1]	24,740,000	[1]	27,305,000	[1]				0.0
Impairment of Real Estate, Attributable to Parent																												$ 4,600,000

[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 $41.7 million and $28.5 million as of December 31, 2013 and 2012, 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.