Noncontrolling Interests (Property Partnerships) (Narrative) (Details) $ in Thousands				12 Months Ended
	Sep. 18, 2015 USD ($) ft²	Oct. 30, 2014 USD ($) ft² floor	May. 31, 2013 USD ($) ft² partners	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 10, 2015 USD ($)	Oct. 09, 2013 USD ($) ft²
Noncontrolling Interest [Line Items]
Property partnerships				$ 1,574,400	$ 1,602,467
Noncontrolling interest in property partnerships recorded upon consolidation				0	0	$ 480,861
Proceeds from sales of real estate				602,600	419,864	250,078
Gains on sales of real estate				$ 375,895	$ 168,039	$ 0
505 9th Street [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Ownership Percentage by Parent	50.00%
Sale Price Of Sold Property	$ 318,000
Mortgage Loan Related to Property Sales	$ 117,000
Net Rentable Area | ft²	322,000
Proceeds from sales of real estate	$ 194,600
Gains on sales of real estate	199,500
Noncontroling interest for a dissolved entity relcassed to accounts payable and and accrued expenses							$ 4,100
Consolidated Entity Excluding Variable Interest Entities (VIE) [Member] | 505 9th Street [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Proceeds from sales of real estate	97,300
Gains on sales of real estate	$ 101,100
767 Fifth Avenue (the General Motors Building)
Noncontrolling Interest [Line Items]
Number of Joint Venture Partners | partners			2
Ownership Percentage			40.00%
Noncontrolling interest in property partnerships recorded upon consolidation			$ 480,861
Net Rentable Area | ft²			1,800,000
Time Square Tower [Member]
Noncontrolling Interest [Line Items]
Sale Price Of Sold Property								$ 684,000
Portion allocation to noncontrolling interest based on carrying value of the total equity of the property prior to the sale								243,500
Difference between net cash proceeds received and noncontrolling interest recognized								429,600
Proceeds from sale of an interest in a real estate asset								$ 673,100
Ownership Percentage								45.00%
Net Rentable Area | ft²								1,246,000
601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member]
Noncontrolling Interest [Line Items]
Sale Price Of Sold Property		$ 1,827,000
Portion allocation to noncontrolling interest based on carrying value of the total equity of the property prior to the sale		849,000
Difference between net cash proceeds received and noncontrolling interest recognized		648,400
Proceeds from sale of an interest in a real estate asset		$ 1,497,000
Ownership Percentage		45.00%
Disposa Group Including Discontinued Operations Secured Debt		$ 712,900
601 Lexington Avenue | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member]
Noncontrolling Interest [Line Items]
Net Rentable Area | ft²		1,669,000
Atlantic Wharf | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member]
Noncontrolling Interest [Line Items]
Net Rentable Area | ft²		791,000
100 Federal Street | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member]
Noncontrolling Interest [Line Items]
Net Rentable Area | ft²		1,323,000
Office Tower [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member]
Noncontrolling Interest [Line Items]
Number of floors in building | floor		59
Low Rise Building [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member]
Noncontrolling Interest [Line Items]
Number of floors in building | floor		6