Investment in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended											12 Months Ended					1 Months Ended
	Dec. 31, 2014 sqft		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 sqft		Dec. 31, 2013		Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2014 sqft		Oct. 31, 2014	Nov. 30, 2014	Mar. 31, 2014		Dec. 31, 2013		Nov. 30, 2012	Oct. 01, 2012	Sep. 30, 2012
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary	$ 146,200,000						$ 310,700,000					$ 146,200,000		$ 310,700,000				$ 146,200,000						$ 310,700,000
Approximate Square Feet (sqft)	33,891,036											33,891,036						33,891,036
Gain on Sale	673,000		16,496,000	1,444,000	104,640,000		7,538,000		(354,000)	(3,583,000)	0	123,253,000		3,601,000		37,053,000
Carrying Value, Net of Discounts and Deferred Origination Fees	1,408,804,000						1,304,839,000					1,408,804,000		1,304,839,000				1,408,804,000						1,304,839,000
100 Park Avenue
General information on each joint venture
Ownership Percentage	49.90%											49.90%						49.90%
Economic Interest (as a percent)	49.90%											49.90%						49.90%
Approximate Square Feet (sqft)	834,000											834,000						834,000
Acquisition Price	95,800,000	[1]										95,800,000	[1]					95,800,000	[1]
717 Fifth Avenue
General information on each joint venture
Ownership Percentage	10.92%	[2]										10.92%	[2]					10.92%	[2]
Economic Interest (as a percent)	10.92%	[2]										10.92%	[2]					10.92%	[2]
Approximate Square Feet (sqft)	119,500	[2]										119,500	[2]					119,500	[2]
Acquisition Price	251,900,000	[1],[2]										251,900,000	[1],[2]					251,900,000	[1],[2]
Proceeds from sale of equity method investments																	67,900,000
Gain on Sale																	3,000,000
Beneficial interest sold (as a percent)																	50.00%
800 Third Avenue
General information on each joint venture
Ownership Percentage	42.95%											42.95%						42.95%
Economic Interest (as a percent)	42.95%											42.95%						42.95%
Approximate Square Feet (sqft)	526,000											526,000						526,000
Acquisition Price	285,000,000	[1]										285,000,000	[1]					285,000,000	[1]
1745 Broadway
General information on each joint venture
Ownership Percentage	56.88%	[3]										56.88%	[3]					56.88%	[3]
Economic Interest (as a percent)	56.88%	[3]										56.88%	[3]					56.88%	[3]
Approximate Square Feet (sqft)	674,000	[3]										674,000	[3]					674,000	[3]
Acquisition Price	520,000,000	[1],[3]										520,000,000	[1],[3]					520,000,000	[1],[3]
Mortgage on properties assumed																		16,000,000
Accrued interest	7,000,000											7,000,000						7,000,000
Jericho Plaza
General information on each joint venture
Ownership Percentage	20.26%											20.26%						20.26%
Economic Interest (as a percent)	20.26%											20.26%						20.26%
Approximate Square Feet (sqft)	640,000											640,000						640,000
Acquisition Price	210,000,000	[1]										210,000,000	[1]					210,000,000	[1]
The Meadows
General information on each joint venture
Ownership Percentage	50.00%											50.00%						50.00%
Economic Interest (as a percent)	50.00%											50.00%						50.00%
Approximate Square Feet (sqft)	582,100											582,100						582,100
Acquisition Price	111,500,000	[1]										111,500,000	[1]					111,500,000	[1]
600 Lexington Avenue
General information on each joint venture
Ownership Percentage	55.00%											55.00%						55.00%
Economic Interest (as a percent)	55.00%											55.00%						55.00%
Approximate Square Feet (sqft)	303,515											303,515						303,515
Acquisition Price	193,000,000	[1]										193,000,000	[1]					193,000,000	[1]
11 West 34th Street
General information on each joint venture
Ownership Percentage	30.00%											30.00%						30.00%
Economic Interest (as a percent)	30.00%											30.00%						30.00%
Approximate Square Feet (sqft)	17,150											17,150						17,150
Acquisition Price	10,800,000	[1]										10,800,000	[1]					10,800,000	[1]
7 Renaissance
General information on each joint venture
Ownership Percentage	50.00%											50.00%						50.00%
Economic Interest (as a percent)	50.00%											50.00%						50.00%
Approximate Square Feet (sqft)	65,641											65,641						65,641
Acquisition Price	4,000,000	[1]										4,000,000	[1]					4,000,000	[1]
3 Columbus Circle
General information on each joint venture
Ownership Percentage	48.90%	[4]										48.90%	[4]					48.90%	[4]
Economic Interest (as a percent)	48.90%	[4]										48.90%	[4]					48.90%	[4]
Approximate Square Feet (sqft)	741,500	[4]										741,500	[4]					741,500	[4]
Acquisition Price	500,000,000	[1],[4]										500,000,000	[1],[4]					500,000,000	[1],[4]
280 Park Avenue
General information on each joint venture
Ownership Percentage	50.00%	[5]										50.00%	[5]					50.00%	[5]
Economic Interest (as a percent)	50.00%	[5]										50.00%	[5]					50.00%	[5]
Approximate Square Feet (sqft)	1,219,158	[5]										1,219,158	[5]					1,219,158	[5]
Acquisition Price	400,000,000	[1],[5]										400,000,000	[1],[5]					400,000,000	[1],[5]
Interest Of Minority Shareholders, Acquired																				11,000,000
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft)	13,045											13,045						13,045
1552-1560 Broadway
General information on each joint venture
Ownership Percentage	50.00%	[6]										50.00%	[6]					50.00%	[6]
Economic Interest (as a percent)	50.00%	[6]										50.00%	[6]					50.00%	[6]
Approximate Square Feet (sqft)	35,897	[6]										35,897	[6]					35,897	[6]
Acquisition Price	136,550,000	[1],[6]										136,550,000	[1],[6]					136,550,000	[1],[6]
724 Fifth Avenue
General information on each joint venture
Ownership Percentage	50.00%											50.00%						50.00%
Economic Interest (as a percent)	50.00%											50.00%						50.00%
Approximate Square Feet (sqft)	65,040											65,040						65,040
Acquisition Price	223,000,000	[1]										223,000,000	[1]					223,000,000	[1]
10 East 53rd Street
General information on each joint venture
Ownership Percentage	55.00%											55.00%						55.00%
Economic Interest (as a percent)	55.00%											55.00%						55.00%
Approximate Square Feet (sqft)	354,300											354,300						354,300
Acquisition Price	252,500,000	[1]										252,500,000	[1]					252,500,000	[1]
33 Beekman
General information on each joint venture
Ownership Percentage	45.90%	[7]										45.90%	[7]					45.90%	[7]
Economic Interest (as a percent)	45.90%	[7]										45.90%	[7]					45.90%	[7]
Approximate Square Feet (sqft)	0	[7]										0	[7]					0	[7]
Acquisition Price	31,000,000	[1],[7]										31,000,000	[1],[7]					31,000,000	[1],[7]
Number of floors of student housing (floors)	30											30						30
521 Fifth Avenue
General information on each joint venture
Ownership Percentage	50.50%	[8]										50.50%	[8]					50.50%	[8]
Economic Interest (as a percent)	50.50%	[8]										50.50%	[8]					50.50%	[8]
Approximate Square Feet (sqft)	460,000	[8]										460,000	[8]					460,000	[8]
Acquisition Price	315,000,000	[1],[8]										315,000,000	[1],[8]					315,000,000	[1],[8]
Gain on Sale														2,800,000
21 East 66th Street
General information on each joint venture
Ownership Percentage	32.28%	[9]										32.28%	[9]					32.28%	[9]
Economic Interest (as a percent)	32.28%	[9]										32.28%	[9]					32.28%	[9]
Approximate Square Feet (sqft)	16,736	[9]										16,736	[9]					16,736	[9]
Acquisition Price	75,000,000	[1],[9]										75,000,000	[1],[9]					75,000,000	[1],[9]
Gain on Sale																				700,000
315 West 36th Street
General information on each joint venture
Ownership Percentage	35.50%											35.50%						35.50%
Economic Interest (as a percent)	35.50%											35.50%						35.50%
Approximate Square Feet (sqft)	147,619											147,619						147,619
Acquisition Price	45,000,000	[1]										45,000,000	[1]					45,000,000	[1]
650 Fifth Avenue
General information on each joint venture
Ownership Percentage	50.00%	[10]										50.00%	[10]					50.00%	[10]
Economic Interest (as a percent)	50.00%	[10]										50.00%	[10]					50.00%	[10]
Approximate Square Feet (sqft)	32,324	[10]										32,324	[10]					32,324	[10]
Acquisition Price	0	[1],[10]										0	[1],[10]					0	[1],[10]
121 Greene Street
General information on each joint venture
Ownership Percentage	50.00%											50.00%						50.00%
Economic Interest (as a percent)	50.00%											50.00%						50.00%
Approximate Square Feet (sqft)	7,131											7,131						7,131
Acquisition Price	27,400,000	[1]										27,400,000	[1]					27,400,000	[1]
175-225 Third Street
General information on each joint venture
Ownership Percentage	95.00%	[11]										95.00%	[11]					95.00%	[11]
Economic Interest (as a percent)	95.00%	[11]										95.00%	[11]					95.00%	[11]
Approximate Square Feet (sqft)	0	[11]										0	[11]					0	[11]
Acquisition Price	74,600,000	[1],[11]										74,600,000	[1],[11]					74,600,000	[1],[11]
55 West 46th Street
General information on each joint venture
Ownership Percentage	25.00%	[12]										25.00%	[12]					25.00%	[12]		25.00%
Economic Interest (as a percent)	25.00%	[12]										25.00%	[12]					25.00%	[12]
Approximate Square Feet (sqft)	347,000	[12]										347,000	[12]					347,000	[12]
Acquisition Price	295,000,000	[1],[12]										295,000,000	[1],[12]					295,000,000	[1],[12]
Partnership interest sold to Harel by joint venture (as a percent)																					75.00%
West Coast office portfolio
General information on each joint venture
Ownership Percentage					42.02%	[13]	42.04%	[13]						42.04%	[13]							42.02%	[13]	42.04%	[13]		27.63%	63.18%
Economic Interest (as a percent)							43.74%							43.74%										43.74%
Gain on Sale																						85,600,000	[13],[14]	2,100,000	[13],[14]
521 Fifth Avenue
General information on each joint venture
Partnership interest sold to Harel by joint venture (as a percent)																										49.50%
Mortgage Loan | Joint venture | 521 Fifth Avenue
General information on each joint venture
Gain on Sale																										19,400,000
Lehman Bros | 1745 Broadway
General information on each joint venture
Remaining percentage of interest acquired in joint venture (percent)																					7.37%
Witkoff | 1745 Broadway
General information on each joint venture
Remaining percentage of interest acquired in joint venture (percent)																		14.79%
Wells Fargo | 1745 Broadway
General information on each joint venture
Remaining percentage of interest acquired in joint venture (percent)																		2.46%
Three Retail and Two Residential Units | 21 East 66th Street
General information on each joint venture
Ownership Percentage	32.28%											32.28%						32.28%
Number of Stores	3											3						3
Number Of Residential Units	2											2						2
Three Residential Units | 21 East 66th Street
General information on each joint venture
Ownership Percentage	16.14%											16.14%						16.14%
Number Of Residential Units	3											3						3
Participating Financing Due March 2016
General information on each joint venture
Loans Receivable, Gross, Commercial, Acquisition	99,600,000				100,000,000							99,600,000						99,600,000				100,000,000
Carrying Value, Net of Discounts and Deferred Origination Fees					40,000,000																	40,000,000
Participating Financing Due March 2016 | Mezzanine loans
General information on each joint venture
Carrying Value, Net of Discounts					60,000,000																	60,000,000
Participating Financing Due February 2022
General information on each joint venture
Loans Receivable, Gross, Commercial, Acquisition	$ 46,200,000											$ 46,200,000						$ 46,200,000		$ 45,800,000

[1] Acquisition price represents the actual or implied gross purchase price for the joint venture.

[2] In June 2012, we recognized $67.9 million of additional cash income, equivalent to profit, due to the distribution of refinancing proceeds and a gain on sale of $3.0 million, on the sale of 50.0% of our remaining interest.

[3] In November 2014, we acquired Lehman Bros.' 7.37% interest in 1745 Broadway. In December 2014, we additionally acquired Witkoff's 14.79% interest and Wells Fargo's 2.46% interest in the property. In connection with the acquisition of Witkoff's interest, we assumed a partner loan of $16.0 million, which is included in mortgages and other loans payable in the consolidated balance sheet, and $7.0 million of accrued interest.

[4] As a result of the sale of a condominium interest in September 2012, Young & Rubicam, Inc., or Y&R, owns a portion of the property, generally floors three through eight referred to as Y&R units. Because the joint venture has an option to repurchase the Y&R units, the gain associated with this sale was deferred.

[5] In October 2014, we, along with Vornado, acquired equally the interest of minority shareholders for $11.0 million.

[6] The purchase price represents only the purchase of the 1552 Broadway interest which comprised approximately 13,045 square feet (unaudited). The joint venture also owns a long-term leasehold interest in the retail space and certain other spaces at 1560 Broadway, which is adjacent to 1552 Broadway.

[7] The property is currently being developed into a 30 story dormitory building, which will be conveyed to Pace University upon its completion under a long-term ground lease arrangement.

[8] In November 2012, we refinanced the previous mortgage and sold 49.5% of our partnership interest. We recognized a gain of $19.4 million on the sale. Due to lack of control, we deconsolidated the entity effective November 30, 2012 and adopted the equity method of accounting thereafter. During the year ended December 31, 2013, we recognized additional post closing costs of $2.8 million as an adjustment to the gain.

[9] In October 2014, the joint venture sold one of the residential units at the property. We recognized a gain of $0.7 million on the sale. As of December 31, 2014, we hold a 32.28% interest in three retail and two residential units at the property and a 16.14% interest in three residential units at the property.

[10] The joint venture owns a long-term leasehold interest in the retail space at 650 Fifth Avenue. In connection with the ground lease obligation, SLG provided a performance guaranty and Sutton executed a contribution agreement to reflect its pro rata obligation. In the event the property is converted into a condominium unit and the landlord elects the purchase option, the joint venture shall be obligated to acquire the unit at the then fair value.

[11] In October 2014, we, along with our joint venture partners, acquired a development site at 175-225 Third Street in Brooklyn, New York. The property is being managed by our joint venture partners. We did not consolidate the property as a result of multiple partners having substantive participating rights.

[12] In October 2014, we acquired the vacant commercial condominium units on floors 2 and 22-34 in the building located at 55 West 46th Street for $295.0 million. We also acquired a retail space on 46th Street and the building’s parking garage and fitness center. We consolidated the property from acquisition through October 31, 2014. In November 2014, we, together with Prudential, formed a joint venture for the ownership of the condominium units. We sold 75.00% of our interest to Prudential and retained 25.00% interest in the joint venture. As a result of Prudential having substantive participating rights, we deconsolidated the property and adopted the equity method of accounting subsequently thereafter.

[13] Prior to the recapitalization, we consolidated the investment for the months of August and September 2012 as a result of our 63.18% ownership interest and control over its activities. Immediately following the recapitalization in September 2012, our investment was 27.63%. The change in ownership resulted in a change in accounting from consolidating the investment to accounting for the joint venture under the equity method of accounting. During the year ended December 31, 2013, the joint venture sold three properties, the proceeds of which were used primarily to repay a portion of the debt. Also during the year ended December 31, 2013, we acquired in aggregate 14.39% ownership interest of two of our joint venture partners. As a result, we had a 42.04% effective ownership interest (43.74% effective economic interest) in the West Coast portfolio as of December 31, 2013. During the year ended December 31, 2014, we sold our ownership interest in the joint venture.

[14] Represents the Company's share of the gain.