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Real Estate Activity During the Three Months Ended March 31, 2013 (Narrative) (Details) (USD $)	3 Months Ended		0 Months Ended		0 Months Ended				3 Months Ended		0 Months Ended	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Feb. 06, 2013 535 Mission [Member] floor sqft	Mar. 22, 2013 Two Patriots Park [Member] sqft	Mar. 26, 2013 Transbay Tower [Member] floor sqft	Feb. 26, 2013 Transbay Tower [Member]	Feb. 07, 2013 Transbay Tower [Member]	Mar. 29, 2013 Reston Land Parcel [Member] sqft	Mar. 31, 2013 Plaza at Almaden [Member]	Mar. 28, 2013 Plaza at Almaden [Member] sqft	Feb. 20, 2013 Montvale Center [Member]	Mar. 31, 2012 Montvale Center [Member]	Mar. 31, 2013 303 Almaden Blvd [Member]	Mar. 28, 2013 303 Almaden Blvd [Member] sqft
Real Estate Properties [Line Items]
Purchase price	$ 289,816,000	$ 621,359,000	$ 71,000,000
Number of floors real estate			27		60
Amount of debt extinguished												25,000,000
Gain on forgiveness of debt from discontinued operations	20,182,000	0									20,200,000
Net Rentable Area			307,000	256,000	1,400,000			250,000		840,000				158,000
Impairment loss from discontinued operations	3,241,000	0											3,200,000
Ownership Percentage					95.00%	95.00%
Payments to Acquire Land					192,000,000			27,000,000
Partners Joint Venture Ownership Percentage						5.00%	50.00%
Contractual Sales Price														40,000,000
Impairment loss	$ 8,306,000	$ 0							$ 8,300,000

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Amount of gain (loss), not previously recognized, resulting from the forgiveness of debt pertaining to a real estate property.

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The charge against earnings resulting from the aggregate write down of all assets from their carrying value to their fair value.

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The cash outflow from the acquisition of a piece of land and anything permanently fixed to it, including buildings, structures on it and so forth, and payments to develop real estate assets by adding improvements on or to a parcel of land. Such improvements may include drainage, utilities, subdividing, access, buildings, and any combination of these elements. Including real estate intended to generate income for the owner; excluding real estate acquired for use by the owner.

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