Mortgage Notes Payable (Narrative) (Details) (USD $)	12 Months Ended							0 Months Ended	12 Months Ended						0 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Fixed Rate Mortgages [Member]	Dec. 31, 2010 Fixed Rate Mortgages [Member]	Dec. 31, 2011 Variable Rate Mortgages [Member]	Dec. 31, 2010 Variable Rate Mortgages [Member]	Jan. 12, 2011 Montvale Center [Member]	Dec. 31, 2011 Minimum [Member] Mortgages [Member]	Dec. 31, 2010 Minimum [Member] Mortgages [Member]	Dec. 31, 2010 Minimum [Member] Variable Rate Mortgages [Member]	Dec. 31, 2010 Maximum [Member] Variable Rate Mortgages [Member]	Dec. 31, 2011 Montvale Center [Member]	Jan. 12, 2011 Montvale Center [Member]	Feb. 01, 2011 Bay Colony Corporate Center [Member]	Aug. 19, 2011 601 Lexington Avenue [Member]	May 11, 2011 601 Lexington Avenue [Member]	Aug. 19, 2011 510 Madison Avenue [Member]	Nov. 09, 2011 Reservoir Place [Member]	Nov. 16, 2011 Atlantic Wharf Development [Member] years
Mortgage Notes Payable [Line Items]
Mortgage notes payable, net	$ 3,123,267,000	$ 3,047,586,000		$ 3,100,000,000	$ 2,700,000,000		$ 317,500,000							$ 25,000,000		$ 725,000,000
Number of real estate assets used to collateralize each loan									1	1
Final maturity date of mortgage notes	April 10, 2022
Contractual interest rate, range minimum				4.75%	5.55%
Contractual interest rate, range maximum				9.93%	7.75%
Contractual interest rate, weighted-average				5.75%	6.18%
Spread on variable rate basis											0.30%	2.20%							2.20%	3.00%
London Interbank Offered Rate						0.30%	0.26%
Debt default, accrued interest rate								9.93%						9.93%
Portion of acquisition included in debt															143,900,000
Amount of mortgage loan that was repaid	1,251,841,000	730,191,000	125,238,000														453,300,000	267,500,000	50,000,000
Interest at a fixed rate															6.53%	4.75%	7.19%
Maturity date								Jun. 06, 2012							Jun. 11, 2012	Apr. 10, 2022	May 11, 2011		Jul. 30, 2014	Apr. 21, 2012
Loss from early extinguishment of debt																				400,000
Cancellation of construction loan facility borrowing capacity																				192,500,000
Number of extension options																				2
Mortgage loan extension options (years)																				1
Outstanding balance of mortgage loan accounted for at fair value	953,100,000	883,400,000													149,200,000
Interest rate used to account for mortgage loan at fair value upon assumption															3.75%
Number of mortgage loans accounted for fair value	6	6
Interest expense decrease due to recording mortgage loans at fair value	9,200,000	3,800,000	4,100,000
Cumulative liability related to fair value adjustments included in mortgage notes payable	23,794,000	27,700,000
Repayment of line of credit		700,000,000
Repayments of line of credit	453,300,000															453,300,000
Description of variable rate basis																			Eurodollar plus 2.20% per annum	LIBOR plus 3.00% per annum
Penalty for early repayment of debt																			0
Loss on extinguishment of debt	(1,494,000)	(89,883,000)	(510,000)																500,000
Basis point increase in variable rate of debt outstanding		1.00%
Increase in interest expense if variable rate increases on debt outstanding		3,200,000
Total real estate	$ 10,746,486,000	$ 10,441,117,000											$ 8,300,000