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Net rentable area for properties owned.

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Unconsolidated Joint Ventures (Details)	3 Months Ended		9 Months Ended
Unconsolidated Joint Ventures (Details)	Sep. 30, 2015 USD ($) ft² room story bldg investment	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) ft² room story bldg investment	Sep. 30, 2014 USD ($)
Liberty Washington LP [Abstract]
Impairment - real estate assets	$ 0	$ 0	$ 16,775,000	$ 0
Land, Buildings and Improvements [Member]
LIberty Venture I LP [Abstract]
Proceeds from Sale of Property	$ 89,784,000		$ 248,116,000
Number of Real Estate Properties Sold \| bldg	6		36
Sold Real Estate Property, Square Feet \| ft²	448,000		2,476,000
Construction in Progress [Member]
Comcast Innovative and Technology Center [Abstract]
Net Rentable Area \| ft²	6,600,000		6,600,000
Total anticipated development cost	$ 643,800,000		$ 643,800,000
Liberty Washington, LP [Member] \| Northern Virginia [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%		25.00%
Liberty Washington, LP [Member] \| Northern Virginia [Member] \| Mortgages [Member]
Liberty Washington LP [Abstract]
Debt Instrument, Debt Default, Amount	$ 46,400,000		$ 46,400,000
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	$ 932,000,000		$ 932,000,000
Anticipated investment in joint ventures	$ 185,000,000		$ 185,000,000
Liberty Venture I, LP [Member] \| New Jersey [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%		25.00%
Liberty Venture I, LP [Member] \| New Jersey [Member] \| Land, Buildings and Improvements [Member]
LIberty Venture I LP [Abstract]
Proceeds from Sale of Property			$ 8,500,000
Number of Real Estate Properties Sold \| bldg			1
Sold Real Estate Property, Square Feet \| ft²			198,000
Gain (Loss) on Investments [Member] \| Company's portion [Member] \| Fair Value, Inputs, Level 2 [Member] \| Liberty Washington, LP [Member] \| Northern Virginia [Member]
Liberty Washington LP [Abstract]
Impairment - real estate assets	$ 3,900,000		$ 11,500,000