Acquisitions (Narratives) (Details)	Sep. 25, 2015 USD ($) ft²	Jul. 31, 2015 USD ($) ft² RenewalOptions shares	Jun. 02, 2015 USD ($) ft²	Mar. 18, 2015 USD ($) ft²	Jan. 20, 2015 USD ($) LimitedPartners	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)
Business Acquisition [Line Items]
Mortgages payable						$ 9,159,413,000	$ 8,263,165,000
Operating Partnership Units Value						$ 80,000,000		$ 0
Real Estate Fund [Member]
Business Acquisition [Line Items]
Number Of Limited Partners | LimitedPartners					1
Retail Properties [Member] | 265 West 34th Street [Member]
Business Acquisition [Line Items]
Square Footage Of Real Estate Property | ft²	1,700
Business Acquisition Cost Of Acquired Entity	$ 28,500,000
Area Of Additional Zoning Air Rights | ft²	15,200
Office Building [Member] | 260 11th Avenue [Member]
Business Acquisition [Line Items]
Square Footage Of Real Estate Property | ft²		235,000
Lease Expiration Date		2021
Area Of Parking Lot And Additional Air Rights | ft²		10,000
Number Of Market Renewal Options | RenewalOptions		2
Term Of Market Renewal Options		5 years
Lease expiration term		99 years
Purchase Options, Land		$ 110,000,000
Annual ground rent		$ 3,900,000
Inflation Rate Agreement For Calculating Annual Ground Rent And Purchase Price Option		Annual ground rent and the purchase option price escalate annually at the lesser of 1.5% or CPI
Operating partnership units issued | shares		813,900
Operating Partnership Units Value		$ 80,000,000
Center Building [Member] | New York Segment [Member] | Office Building [Member]
Business Acquisition [Line Items]
Square Footage Of Real Estate Property | ft²				437,000
Business Acquisition Cost Of Acquired Entity				$ 142,000,000
Mortgages payable				$ 62,000,000
Interest Rate, End of Period (in percentage)				4.43%
Debt Instrument Maturity				October 2018
Crowne Plaza Time Square Hotel [Member]
Business Acquisition [Line Items]
Equity method ownership percentage					33.00%		11.00%
Crowne Plaza Time Square Hotel [Member] | Real Estate Fund Joint Venture [Member]
Business Acquisition [Line Items]
Equity method ownership percentage					57.00%
Business Acquisition Cost Of Acquired Entity					$ 95,000,000
Mortgages payable					$ 310,000,000
Debt Instrument Maturity					December 2018
150 West 34th Street [Member] | New York Segment [Member] | Retail Properties [Member]
Business Acquisition [Line Items]
Square Footage Of Real Estate Property | ft²			78,000
Business Acquisition Cost Of Acquired Entity			$ 355,000,000
Mortgages payable			$ 205,000,000
Lease Expiration Date			May 2019
Area Of Additional Zoning Air Rights | ft²			226,000