Real Estate Activity During the Six Months Ended June 30, 2013 (Narrative) (Details) (USD $)	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
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Feb. 06, 2013 535 Mission [Member] floor sqft	Mar. 22, 2013 Two Patriots Park [Member] sqft	Apr. 25, 2013 Siz Zero One Massachusetts Avenue [Member] sqft	Jun. 14, 2013 Seventeen Cambridge Center [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]	Jun. 30, 2012 Montvale Center [Member]	Jun. 30, 2013 303 Almaden Blvd [Member]	Mar. 31, 2013 303 Almaden Blvd [Member]	Mar. 28, 2013 303 Almaden Blvd [Member] sqft	Jun. 30, 2013 Mountain View Research And Technology Parks [Member]	Apr. 10, 2013 Mountain View Research And Technology Parks [Member]	Apr. 10, 2013 Mountain View Research [Member] Buildings sqft	Apr. 10, 2013 Mountain View Technology [Member] Buildings sqft	May 31, 2013 767 Fifth Avenue (The GM Building) [Member] partners sqft	Jun. 30, 2013 767 Fifth Avenue (The GM Building) [Member]
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