Mortgages and Other Loans Payable (Details) (USD $)	3 Months Ended								12 Months Ended								12 Months Ended																																																						1 Months Ended										1 Months Ended						12 Months Ended				0 Months Ended											2 Months Ended	9 Months Ended	1 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Nov. 30, 2012	Dec. 31, 2013 609 Partners, LLC		Dec. 31, 2012 609 Partners, LLC		Dec. 31, 2013 609 Partners, LLC Series E preferred units	Dec. 31, 2013 125 Park Avenue		Dec. 31, 2012 125 Park Avenue	Dec. 31, 2013 711 Third Avenue		Dec. 31, 2012 711 Third Avenue	Dec. 31, 2013 625 Madison Avenue		Dec. 31, 2012 625 Madison Avenue	Dec. 31, 2013 500 West Putnam		Dec. 31, 2012 500 West Putnam	Dec. 31, 2013 420 Lexington Avenue		Dec. 31, 2012 420 Lexington Avenue	Dec. 31, 2013 Landmark Square		Dec. 31, 2012 Landmark Square	Dec. 31, 2013 485 Lexington Avenue		Dec. 31, 2012 485 Lexington Avenue	Dec. 31, 2013 120 West 45th Street		Dec. 31, 2012 120 West 45th Street	Dec. 31, 2013 762 Madison Avenue		Dec. 31, 2012 762 Madison Avenue	Dec. 31, 2013 2 Herald Square		Dec. 31, 2012 2 Herald Square	Dec. 31, 2013 885 Third Avenue		Dec. 31, 2012 885 Third Avenue	Dec. 31, 2013 Other loan payable		Dec. 31, 2012 Other loan payable		Dec. 31, 2013 One Madison Avenue		Dec. 31, 2012 One Madison Avenue	Dec. 31, 2013 100 Church		Dec. 31, 2012 100 Church	Dec. 31, 2013 919 Third Avenue		Dec. 31, 2012 919 Third Avenue		Dec. 31, 2013 400 East 57th Street		Dec. 31, 2012 400 East 57th Street	Dec. 31, 2013 400 East 58th Street		Dec. 31, 2012 400 East 58th Street	Feb. 28, 2013 1515 Broadway	Apr. 30, 2012 1515 Broadway	Dec. 31, 2013 1515 Broadway		Dec. 31, 2012 1515 Broadway		Dec. 31, 2013 300 Main Street		Dec. 31, 2012 300 Main Street		Oct. 31, 2013 220 East 42nd Street	Aug. 30, 2013 220 East 42nd Street	Dec. 31, 2013 220 East 42nd Street		Dec. 31, 2012 220 East 42nd Street		Dec. 31, 2013 Master repurchase		Dec. 31, 2012 Master repurchase		Dec. 31, 2013 16 Court Street		Dec. 31, 2012 16 Court Street		Dec. 31, 2013 180 Maiden Lane		Dec. 31, 2012 180 Maiden Lane		Dec. 31, 2013 248-252 Bedford Avenue		Dec. 31, 2012 248-252 Bedford Avenue	Nov. 30, 2013 LIBOR Master repurchase	Sep. 30, 2013 LIBOR Master repurchase	Dec. 31, 2013 LIBOR 16 Court Street	Dec. 31, 2013 LIBOR Minimum Master repurchase	Dec. 31, 2013 LIBOR Maximum Master repurchase
Mortgages and Other Loans Payable
Interest rate, fixed rate debt (as a percent)													5.00%	[1],[2]				5.75%	[1]		4.99%	[1]		7.27%	[1]		5.52%	[1]		7.15%	[1]		4.00%	[1]		5.61%	[1]		6.12%	[1]		3.75%	[1]		5.36%	[1]		6.26%	[1]		8.00%	[1],[3]			5.91%	[1]		4.68%	[1]		5.12%	[1],[4]			4.13%	[1]		4.13%	[1]				3.93%	[1],[5]			0.00%	[1],[6]					0.00%	[1],[7]
Interest rate, floating rate debt (as a percent)																																																																									0.00%	[1],[5]									4.05%	[1],[7]			3.28%	[1],[8]			3.22%	[1],[9]			2.39%	[1],[10]			2.44%	[1]
Total fixed rate debt	$ 4,130,629,000				$ 3,457,769,000				$ 4,130,629,000	$ 3,457,769,000			$ 23,000	[2]	$ 23,000	[2]		$ 146,250,000		$ 146,250,000	$ 120,000,000		$ 120,000,000	$ 120,830,000		$ 125,603,000	$ 23,529,000		$ 24,060,000	$ 182,641,000		$ 184,992,000	$ 82,909,000		$ 84,486,000	$ 450,000,000		$ 450,000,000	$ 170,000,000		$ 170,000,000	$ 8,211,000		$ 8,371,000	$ 191,250,000		$ 191,250,000	$ 267,650,000		$ 267,650,000	$ 50,000,000	[3]	$ 50,000,000	[3]	$ 587,336,000		$ 607,678,000	$ 230,000,000		$ 230,000,000	$ 500,000,000	[4]	$ 500,000,000	[4]	$ 70,000,000		$ 70,000,000	$ 30,000,000		$ 30,000,000			$ 900,000,000	[5]	$ 0	[5]	$ 0	[6]	$ 11,500,000	[6]			$ 0	[7]	$ 185,906,000	[7]
Total floating rate debt	729,949,000				1,157,695,000				729,949,000	1,157,695,000																																																															0	[5]	769,813,000	[5]							275,000,000	[7]	0	[7]	91,000,000	[8]	116,667,000	[8]	79,243,000	[9]	0	[9]	262,706,000	[10]	271,215,000	[10]	22,000,000		0
Total mortgages and other loans payable	4,860,578,000				4,615,464,000				4,860,578,000	4,615,464,000
Preferred Units, shares issued (shares)																	63,900,000
Preferred Units (as a percent)																	5.00%
Preferred Units, liquidation preference (in dollars per share)																	$ 1.00
Preferred Units, shares outstanding (shares)																	22,658
Interest in property (as a percent)																																																													51.00%
Extinguishment of debt																																																																							775,000,000	447,200,000										185,900,000					175,000,000
Refinanced mortgage loan																																																																							900,000,000.0	775,000,000.0									275,000,000.0
Term																																																																							12 years	7 years									7 years
Loss (gain) on early extinguishment of debt	(5,000)	0	10,000	18,513,000	6,978,000	0	0	0	18,518,000	6,978,000	(904,000)																																																												18,500,000
Prepayment penalty																																																																							7,600,000
Revolving credit facility, maximum borrowing capacity	300,000,000.0								300,000,000.0			1,600,000,000.0
Interest rate added to base rate (as a percent)																																																																																																						3.50%	3.00%	3.50%	2.50%	3.25%
Repaid loan																																																																																											5,000,000
Gross book value of the properties and debt and preferred equity investments collateralizing the mortgages and other loans payable	$ 8,000,000,000				$ 7,600,000,000				$ 8,000,000,000	$ 7,600,000,000
Variable rate floor (percent)																																																																																																								0.50%

[1] Effective weighted average interest rate for the year ended December 31, 2013, taking into account interest rate hedges in effect during the period.

[2] As part of an acquisition, the Operating Partnership issued 63.9 million units of its 5.0% Series E preferred units, or the Series E units, with a liquidation preference of $1.00 per unit. As of December 31, 2013, 22,658 Series E units remaining outstanding.

[3] This loan is secured by a portion of a preferred equity investment.

[4] We own a 51.0% controlling interest in the joint venture that is the borrower on this loan. This loan is non-recourse to us.

[5] In April 2012, we refinanced the previous $447.2 million mortgage with a $775.0 million seven-year mortgage. In February 2013, we refinanced the previous $775.0 million mortgage with a new $900.0 million 12-year mortgage and realized a net loss on early extinguishment of debt of approximately $18.5 million, including a prepayment penalty of $7.6 million.

[6] The property was sold in September 2013.

[7] In October 2013, we closed on a $275.0 million seven-year mortgage. This new mortgage replaced the $185.9 million previous mortgage that was repaid in August 2013.

[8] In December 2013, we entered into a new Master Repurchase Agreement, or MRA, with a maximum facility capacity of $300.0 million, under which we agreed to sell certain debt investments in exchange for cash with a simultaneous agreement to repurchase the same debt investments at a certain date or on demand. This new MRA, which bears interest ranging from 250 and 325 basis points over one-month LIBOR depending on the pledged collateral, replaced the previous $175.0 million MRA facility, which bore interest based on one-month LIBOR plus 300 basis points basis points through September 2013 and 350 basis points through November 2013.

[9] In April 2013, we acquired interests from our joint venture partner, CIF, and have consolidated the entity due to our controlling interest. In December 2013, we repaid $5.0 million of the principal balance and modified and extended the terms of the existing mortgage. The modified mortgage bears interest based on one-month LIBOR plus 350 basis points with a floor of 50 basis points.

[10] In connection with this consolidated joint venture obligation, we executed a master lease agreement. Our partner has executed a contribution agreement to reflect its pro rata share of the obligation under the master lease.