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Real Estate Activity During the Six Months Ended June 30, 2013 (Narrative) (Details) (USD $)
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3 Months Ended | 6 Months Ended | 0 Months Ended | 0 Months Ended | 3 Months Ended | 0 Months Ended | 6 Months Ended | 3 Months Ended | 3 Months Ended | 0 Months Ended | 1 Months Ended | ||||||||||||||
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Jun. 30, 2013
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Jun. 30, 2012
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Jun. 30, 2013
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Jun. 30, 2012
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Feb. 06, 2013
535 Mission [Member]
floor
sqft
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Mar. 22, 2013
Two Patriots Park [Member]
sqft
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Apr. 25, 2013
Siz Zero One Massachusetts Avenue [Member]
sqft
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Jun. 14, 2013
Seventeen Cambridge Center [Member]
sqft
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Mar. 26, 2013
Transbay Tower [Member]
floor
sqft
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Feb. 26, 2013
Transbay Tower [Member]
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Feb. 07, 2013
Transbay Tower [Member]
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Mar. 29, 2013
Reston Land Parcel [Member]
sqft
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Mar. 31, 2013
Plaza at Almaden [Member]
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Mar. 28, 2013
Plaza at Almaden [Member]
sqft
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Feb. 20, 2013
Montvale Center [Member]
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Jun. 30, 2012
Montvale Center [Member]
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Jun. 30, 2013
303 Almaden Blvd [Member]
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Mar. 31, 2013
303 Almaden Blvd [Member]
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Mar. 28, 2013
303 Almaden Blvd [Member]
sqft
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Jun. 30, 2013
Mountain View Research And Technology Parks [Member]
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Apr. 10, 2013
Mountain View Research And Technology Parks [Member]
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Apr. 10, 2013
Mountain View Research [Member]
Buildings
sqft
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Apr. 10, 2013
Mountain View Technology [Member]
Buildings
sqft
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May 31, 2013
767 Fifth Avenue (The GM Building) [Member]
partners
sqft
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Jun. 30, 2013
767 Fifth Avenue (The GM Building) [Member]
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| Real Estate Properties [Line Items] | |||||||||||||||||||||||||
| Revenues | $ 515,320,000 | $ 471,349,000 | $ 998,334,000 | $ 915,042,000 | $ 1,529,000 | $ 5,200,000 | $ 22,600,000 | ||||||||||||||||||
| Operating income | 170,815,000 | 173,319,000 | 301,250,000 | 316,886,000 | 873,000 | (300,000) | 400,000 | ||||||||||||||||||
| Number of Joint Venture Partners | 2 | ||||||||||||||||||||||||
| Purchase price | 522,900,000 | 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 | 0 | 0 | 20,182,000 | 0 | 20,200,000 | 17,800,000 | 0 | ||||||||||||||||||
| Net Rentable Area | 307,000 | 256,000 | 478,000 | 195,000 | 1,400,000 | 250,000 | 840,000 | 158,000 | 604,000 | 135,000 | 1,800,000 | ||||||||||||||
| Aggregate purchase price | 233,500,000 | ||||||||||||||||||||||||
| Impairment loss from discontinued operations | 0 | 0 | 3,241,000 | 0 | 0 | 3,200,000 | |||||||||||||||||||
| Ownership Percentage | 95.00% | 95.00% | 40.00% | ||||||||||||||||||||||
| 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 | $ 0 | $ 0 | $ 8,306,000 | $ 0 | $ 8,300,000 | ||||||||||||||||||||
| Number of real estate properties | 16 | 7 | |||||||||||||||||||||||