Subsequent Events (Details 1) ft² in Thousands		12 Months Ended
	Feb. 08, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Feb. 11, 2016
Subsequent Events [Line Items]
Net proceeds from borrowings		$ 4,468,872,000	$ 2,428,285,000	$ 2,262,245,000
Mortgages payable, net		$ 9,513,713,000	$ 8,187,843,000
770 Broadway [Member]
Subsequent Events [Line Items]
Interest Rate, End of Period (in percentage)		5.65%
Mortgages payable, net		$ 353,000,000
Debt Instrument Maturity		March 2016
770 Broadway [Member] | Subsequent Event [Member]
Subsequent Events [Line Items]
Notes And Loans Payable Refinanced Amount (in US Dollars)	$ 700,000,000
Square Footage Of Real Estate Property | ft²	1,158
Debt Term	5 years
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	1.75%
Interest Rate, End of Period (in percentage)					2.18%
Net proceeds from borrowings	$ 330,000,000
770 Broadway [Member] | Subsequent Event [Member] | Fixed Rate [Member]
Subsequent Events [Line Items]
Debt Term	4 years 6 months
Interest Rate, End of Period (in percentage)	2.56%