Debt (Narratives) (Details) (USD $)	3 Months Ended						0 Months Ended			0 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Mar. 31, 2014 Notes And Mortgages Payable Fixed Rate [Member]	Dec. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member]	Mar. 31, 2014 Notes And Mortgages Payable Fixed Rate [Member] New York 909 Third Avenue [Member]	Apr. 16, 2014 Notes And Mortgages Payable Fixed Rate [Member] New York 909 Third Avenue [Member] Subsequent Event [Member] sqft	Mar. 31, 2014 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2013 Notes And Mortgages Payable Variable Rate [Member]	Jan. 31, 2014 Notes And Mortgages Payable Variable Rate [Member] New York Manhattan 220 Central Park South Site [Member] Extensions	Mar. 31, 2014 Notes And Mortgages Payable Variable Rate [Member] New York Manhattan 220 Central Park South Site [Member]
Debt Instrument [Line Items]
Mortgages payable	$ 8,913,358,000		$ 8,331,993,000	$ 7,546,030,000	$ 7,563,133,000	$ 193,000,000		$ 1,367,328,000	$ 768,860,000	$ 600,000,000
Notes And Loans Payable Refinanced Amount							350,000,000
Debt Term							7 years
Interest Rate, End of Period (in percentage)				4.56%		5.64%	3.91%	2.55%			2.90%
Proceeds from borrowings	$ 600,000,000	$ 1,499,375,000					$ 145,000,000
Square Footage Of Real Estate Property							1,300,000
Length Of Extension Available										1 year
Number Of Extensions Available										3
Debt Instrument Maturity							May 2021			January 2016
Spread Over LIBOR (in percentage)										2.75%