Debt (Narratives) (Details) (USD $)	12 Months Ended					0 Months Ended			1 Months Ended	12 Months Ended			0 Months Ended							0 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member]	Feb. 20, 2013 Notes And Mortgages Payable Fixed Rate [Member] New York Office 666 Fifth Avenue [Member]	Nov. 27, 2013 Notes And Mortgages Payable Fixed Rate [Member] New York Eleven Penn Plaza [Member] sqft	Oct. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Oct. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member] Tranches	Dec. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Oct. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Skyline Properties Senior Position [Member]	Oct. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Skyline Properties Junior Position [Member]	Mar. 25, 2013 Notes And Mortgages Payable Fixed Rate [Member] Retail Bergen Town Center [Member] sqft	Dec. 31, 2013 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2013 Total Notes And Mortgages Payable [Member]	Dec. 31, 2012 Total Notes And Mortgages Payable [Member]	Dec. 31, 2013 Senior Unsecured Notes [Member]	Dec. 31, 2012 Senior Unsecured Notes [Member]	Mar. 28, 2013 Unsecured Revolving Credit Facilities [Member] Extensions CreditFacilities	Dec. 31, 2013 Unsecured Revolving Credit Facilities [Member]	Dec. 31, 2012 Unsecured Revolving Credit Facilities [Member]
Debt Instrument [Line Items]
Notes And Loans Payable Refinanced Amount						$ 390,000,000	$ 450,000,000						$ 300,000,000
Debt Term						10 years	7 years						10 years
Interest Rate, End of Period (in percentage)				4.56%		3.61%	3.95%	5.74%	5.74%		2.965%		3.56%	2.28%		4.35%		5.69%			1.32%
Proceeds from borrowings	2,262,245,000	3,593,000,000	3,412,897,000			387,000,000	107,000,000
Square Footage Of Real Estate Property							1,100,000						948,000
Mortgages payable	8,331,993,000	8,599,222,000		7,563,133,000	6,771,001,000								282,312,000	768,860,000	1,828,221,000	8,331,993,000	8,599,222,000
Face amount of mortgage loan								678,000,000	678,000,000		350,000,000	328,000,000
Number Of Loan Tranches									2
Maturity, Range, Start									1.25	February 2017										June 2015
Maturity, Range, End									February 2022											June 2017
Length Of Extension Available								1 year												6 months
Line of Credit Facility, Maximum Borrowing Capacity																				1,250,000,000
Number Of Revolving Credit Facilities																				2
Extension Executed On Number Of Revolving Credit Facility																				1
Number Of Extensions Available																				2
Debt Instrument, Description of Variable Rate Basis																				LIBOR
Spread Over LIBOR (in percentage)																				1.35%	1.15%
Facility Fee (in percentage)																				0.30%	0.20%
Senior unsecured notes	1,350,855,000	1,358,008,000																1,350,855,000	1,358,008,000
Revolving credit facility debt	295,870,000	1,170,000,000																			295,870,000	1,170,000,000
Collateral Property Net Carrying Value	$ 9,300,000,000