Real Estate Activity During the Nine Months Ended September 30, 2014 (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended															0 Months Ended				0 Months Ended							0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Jul. 29, 2014 Mountain View Technology Park and Mountain View Research Building Sixteen [Member]	Aug. 20, 2014 Land at One Reston Overlook [Member]	Aug. 22, 2014 Land Parcel Within Broad Run Business Park [Member]	Aug. 22, 2014 Land Parcel Within Broad Run Business Park [Member] acre	Jun. 11, 2014 Patriots park [Member] Buildings sqft	May 20, 2014 10 CityPoint [Member] sqft	May 20, 2014 888 Boylston Street [Member] sqft	Apr. 03, 2014 690 Folsom Street [Member] sqft	Apr. 01, 2014 99 Third Avenue [Member] sqft	Feb. 10, 2014 The Avant at Reston Town Center Residential [Member] apartments sqft	Jul. 29, 2014 Mountain View Technology [Member] Mountain View Technology Park and Mountain View Research Building Sixteen [Member] Buildings sqft	Jul. 29, 2014 Mountain View Research Building Sixteen [Member] Mountain View Technology Park and Mountain View Research Building Sixteen [Member] sqft	Aug. 31, 2014 250 West 55th Street [Member] sqft	Sep. 17, 2014 680 Folsom Street [Member] sqft	Sep. 16, 2014 Consolidated Properties [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member]	Apr. 10, 2014 Consolidated Properties [Member] Salesforece Tower (formerly Transbay Tower) [Member] sqft floor	Sep. 16, 2014 Consolidated Properties [Member] 601 Lexington Avenue [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] sqft	Sep. 16, 2014 Consolidated Properties [Member] Atlantic Wharf Office [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] sqft	Sep. 16, 2014 Consolidated Properties [Member] 100 Federal Street [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] sqft	Sep. 16, 2014 Consolidated Properties [Member] Low Rise [Member] 601 Lexington Avenue [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] floor	Sep. 16, 2014 Consolidated Properties [Member] Office Tower [Member] 601 Lexington Avenue [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] floor	Oct. 02, 2014 Subsequent Event [Member] Patriots park [Member] sqft Buildings	Oct. 30, 2014 Subsequent Event [Member] Consolidated Properties [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member]	Oct. 30, 2014 Subsequent Event [Member] Consolidated Properties [Member] 601 Lexington Avenue [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] sqft	Oct. 30, 2014 Subsequent Event [Member] Consolidated Properties [Member] Atlantic Wharf Office [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] sqft	Oct. 30, 2014 Subsequent Event [Member] Consolidated Properties [Member] 100 Federal Street [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] sqft	Oct. 30, 2014 Subsequent Event [Member] Consolidated Properties [Member] Low Rise [Member] 601 Lexington Avenue [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] floor	Oct. 30, 2014 Subsequent Event [Member] Consolidated Properties [Member] Office Tower [Member] 601 Lexington Avenue [Member] 601 Lexington Avenue, Atlantic Wharf Office Building and One Hundred Federal Street [Member] floor
Real Estate Properties [Line Items]
Number of real estate properties									3						7											3
Number of apartment units														359
Area of Real Estate Property									706,000	245,000	425,000	26,000	17,000	355,000	135,000	63,000	988,000	525,000		1,400,000	1,669,000	791,000	1,323,000			706,000		1,669,000	791,000	1,323,000
Sales price					$ 92,100,000			$ 9,800,000	$ 321,000,000																	$ 321,000,000
Ownership Percentage by Parent																				95.00%
Ownership Percentage																			45.00%								45.00%
Square Footage Of Signed Lease																				714,000
Number of floors real estate																				61				6	59						6	59
Secured Debt Carrying Value																			712,900,000								712,900,000
Proceeds from the sales of real estate			103,542,000	160,815,000	91,200,000	2,600,000	9,700,000
Gains on sales of real estate	41,937,000	0	41,937,000	0	36,400,000	1,200,000	4,300,000
Area of Land								15.5
Gross Sales Price Before Deducting Indebtedness																			$ 1,827,000,000								$ 1,827,000,000