Unconsolidated Joint Ventures (Details) - Jun. 30, 2015 $ in Thousands	USD ($) ft² room story bldg investment	USD ($) ft² room story bldg investment
Land, Buildings and Improvements [Member]
LIberty Venture I LP [Abstract]
Proceeds from Sale of Property	$ 116,100	$ 158,332
Number of Real Estate Properties Sold | bldg	23	30
Sold Real Estate Property, Square Feet | ft²	1,406,000	2,028,000
Construction in Progress [Member]
Comcast Innovative and Technology Center [Abstract]
Net Rentable Area | ft²	3,000,000	3,000,000
Total anticipated development cost	$ 361,200	$ 361,200
Liberty Washington, LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%	25.00%
Liberty Washington, LP [Member] | Northern Virginia [Member]
Liberty Washington LP [Abstract]
Impairment of Real Estate, Attributable to Parent	$ 7,600	$ 7,600
Comcast Innovation and Technology Center [Member] | Construction in Progress [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%	20.00%
Comcast Innovative and Technology Center [Abstract]
Number of equity method investments | investment	2	2
Number of stories in a high-rise building | story	59	59
Net Rentable Area | ft²	1,300,000	1,300,000
Number of hotel rooms | room	222	222
Total anticipated development cost	$ 932,000	$ 932,000
Anticipated investment in joint ventures	$ 185,000	$ 185,000
Liberty Venture I, LP [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%	25.00%
Liberty Venture I, LP [Member] | Land, Buildings and Improvements [Member]
LIberty Venture I LP [Abstract]
Proceeds from Sale of Property	$ 8,500	$ 8,500
Number of Real Estate Properties Sold | bldg	1	1
Sold Real Estate Property, Square Feet | ft²	198,000	198,000