Investments in Unconsolidated Joint Ventures Investment in Unconsolidated Joint Ventures (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended				0 Months Ended				0 Months Ended		0 Months Ended			0 Months Ended			3 Months Ended	9 Months Ended			0 Months Ended												3 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	May 30, 2013 125 West55th Street [Member]	Apr. 10, 2013 125 West55th Street [Member] sqft	Sep. 30, 2013 Boston Properties Office Value-Added Fund, L.P. [Member]		Apr. 10, 2013 Mountain View Research And Technology Parks [Member]	Apr. 10, 2013 Mountain View Technology [Member]	Apr. 10, 2013 Mountain View Research [Member]	Feb. 28, 2013 Annapolis Junction Building Six [Member] sqft	May 31, 2013 500 North Capitol [Member]	Sep. 30, 2013 500 North Capitol [Member] sqft	Oct. 29, 2013 Annapolis Junction Building Seven [Member] sqft	Apr. 04, 2013 Annapolis Junction Building Seven [Member] Y	Sep. 26, 2013 Annapolis Junction Building Seven [Member] sqft	Jun. 05, 2013 540 Madison Avenue [Member]	Jul. 19, 2013 8th Avenue and 46th Street [Member]	May 31, 2013 767 Fifth Avenue (The GM Building) [Member] partners sqft	Sep. 30, 2013 767 Fifth Avenue (The GM Building) [Member]	Sep. 30, 2012 767 Fifth Avenue (The GM Building) [Member]	Sep. 30, 2012 Mountain View Research And Technology Parks [Member]	Apr. 10, 2013 Mountain View Research And Technology Parks [Member]	May 31, 2013 Construction Loans [Member] 500 North Capitol [Member]	Apr. 10, 2013 Mountain View Technology [Member]	Apr. 10, 2013 Mountain View Research [Member]	May 31, 2013 First Mortgage [Member] 500 North Capitol [Member]	Jun. 05, 2013 First Mortgage [Member] 540 Madison Avenue [Member]	Jun. 05, 2013 Prior to refinance first mortgage loan [Member] 540 Madison Avenue [Member]	Apr. 10, 2013 Company [Member]	May 30, 2013 Company [Member] 125 West55th Street [Member]	Jul. 19, 2013 Company [Member] 8th Avenue and 46th Street [Member]	May 31, 2013 Company [Member] Construction Loans [Member] 500 North Capitol [Member]	Jun. 05, 2013 Company [Member] Prior to refinance first mortgage loan [Member] 540 Madison Avenue [Member]	Apr. 10, 2013 Mountain View Research And Technology Parks [Member]	Sep. 30, 2013 Value Added Fund [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Description of Variable Rate Basis										LIBOR plus 2.50%	LIBOR plus 2.00%		LIBOR plus 1.65%		LIBOR plus 1.65%			LIBOR plus 1.50%
Equity Method Investment, Ownership Percentage						60.00%	39.50%	[1]				50.00%	30.00%	30.00%	50.00%	50.00%	50.00%	60.00%	50.00%																	39.50%
Net Rentable Area						588,000						119,000		231,000	119,000		125,000			1,800,000
Construction Loan Facility Borrowing Capacity																$ 22,000,000
Debt Instrument, Basis Spread on Variable Rate										2.50%	2.00%				1.65%	1.65%									1.65%				1.50%
Maturity date					Mar. 10, 2020					Nov. 22, 2014	May 31, 2014				Nov. 17, 2014	Apr. 04, 2016									Oct. 14, 2014			Jun. 06, 2023	Jun. 05, 2018	Jul. 11, 2013
Proceeds from Sale of Equity Method Investments			223,067,000	3,057,000	253,700,000																											152,200,000
Losses from Early Extinguishment of Debt	(30,000)	(5,494,000)	122,000	(4,453,000)					(400,000)																(1,000,000)					(300,000)	(200,000)			(300,000)	(200,000)
Number of extensions																2
Extension Option (in years)																1
Aggregate purchase price																								233,100,000
Repayments of Secured Debt			96,750,000	248,304,000																					90,600,000	20,000,000	90,000,000			118,000,000
Repayments of Notes Payable																										3,700,000	8,600,000
Ownerhip upon acquistion																																				100.00%
Gains on Consolidation of Joint Ventures	1,810,000	0	(385,991,000)	0					(26,500,000)											(359,500,000)	3,900,000	(359,500,000)	(26,500,000)														(2,100,000)
Contractual Sales Price						470,000,000
Carrying Value of Debt Assumed in Sale					198,600,000
Gain on sale of real estate					43,200,000														12,600,000														11,300,000
Number of Joint Venture Partners																				2
Proceeds from Issuance of Secured Debt																												105,000,000	120,000,000
Coupon/Stated Rate (percent)					6.09%																							4.15%		5.20%
Imputed Sales Price																			45,000,000
Equity Method Investment, Net Sales Proceeds																			$ 21,800,000

[1] The Company acquired Mountain View Research Park and Mountain View Technology Park from the Value-Added Fund on April 10, 2013 (See Note 3). As of September 30, 2013, the investment is comprised of undistributed cash.