Investment in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended								12 Months Ended									1 Months Ended																					1 Months Ended			1 Months Ended							1 Months Ended										1 Months Ended		12 Months Ended			1 Months Ended	12 Months Ended											1 Months Ended				1 Months Ended		12 Months Ended	1 Months Ended	12 Months Ended
	Dec. 31, 2013 sqft	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 investment sqft	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 100 Park Avenue sqft		Dec. 31, 2013 21 West 34th Street sqft		Dec. 31, 2013 1604-1610 Broadway sqft		Jun. 30, 2012 717 Fifth Avenue	Dec. 31, 2013 717 Fifth Avenue sqft		Dec. 31, 2013 800 Third Avenue sqft		Dec. 31, 2013 1745 Broadway sqft		Dec. 31, 2013 1 and 2 Jericho Plaza sqft		Dec. 31, 2013 The Meadows sqft		Dec. 31, 2013 388 and 390 Greenwich Street sqft		Dec. 31, 2013 180/182 Broadway sqft		Dec. 31, 2013 600 Lexington Avenue sqft		Dec. 31, 2013 11 West 34th Street sqft		Dec. 31, 2013 7 Renaissance sqft		Sep. 30, 2012 3 Columbus Circle	Dec. 31, 2013 3 Columbus Circle sqft		Mar. 31, 2011 280 Park Avenue	Dec. 31, 2013 280 Park Avenue sqft		May 31, 2011 280 Park Avenue	Dec. 31, 2013 1552-1560 Broadway sqft		Dec. 31, 2012 1552-1560 Broadway sqft	Oct. 31, 2013 747 Madison Avenue unit	Dec. 31, 2013 747 Madison Avenue sqft		Sep. 30, 2013 747 Madison Avenue unit	Dec. 31, 2013 724 Fifth Avenue sqft		Dec. 31, 2013 10 East 53rd Street sqft		Dec. 31, 2013 33 Beekman sqft floor		Feb. 28, 2013 West Coast office portfolio	Sep. 30, 2012 West Coast office portfolio sqft property	Dec. 31, 2013 West Coast office portfolio property sqft		Aug. 31, 2012 West Coast office portfolio	Nov. 30, 2012 521 Fifth Avenue	Dec. 31, 2013 521 Fifth Avenue sqft		Dec. 31, 2013 21 East 66th Street sqft		Dec. 31, 2013 21 East 66th Street Three retail units unit	Dec. 31, 2013 21 East 66th Street 2 residential units unit	Dec. 31, 2013 21 East 66th Street Four residential units unit	Dec. 31, 2013 315 West 36th Street sqft		Dec. 31, 2013 Herald Center sqft		Nov. 30, 2013 650 Fifth Avenue	Dec. 31, 2013 650 Fifth Avenue sqft		Dec. 31, 2013 Joint venture 747 Madison Avenue	Nov. 30, 2012 521 Fifth Avenue	Nov. 30, 2012 521 Fifth Avenue Joint venture	Dec. 31, 2013 Mezzanine loans Joint venture West Coast office portfolio	Nov. 30, 2012 Mortgage Loan Joint venture West Coast office portfolio	Dec. 31, 2013 Mortgage Loan Joint venture West Coast office portfolio
General information on each joint venture
Number of VIEs in which the entity is not primary beneficiary									5
Net equity investment in VIEs in which the entity is not primary beneficiary	$ 310,700,000				$ 117,700,000				$ 310,700,000	$ 117,700,000
Ownership Interest (as a percent)												49.90%		50.00%		70.00%	[1]		10.92%	[2]	42.95%		32.26%		20.26%		50.00%		50.60%	[3]	25.50%	[4]	55.00%		30.00%	[5]	50.00%			48.90%	[6]		50.00%	[7]		50.00%	[8]			33.33%	[9]		50.00%		55.00%		45.90%	[10]			42.02%	[11]			50.50%	[12]	32.28%	[13]	32.28%		16.14%	35.50%		40.00%	[14]		50.00%	[15]	100.00%
Economic Interest (as a percent)												49.90%		50.00%		70.00%	[1]		10.92%	[2]	42.95%		32.26%		20.26%		50.00%		50.60%	[3]	25.50%	[4]	55.00%		30.00%	[5]	50.00%			48.90%	[6]		49.50%	[7]		50.00%	[8]			33.33%	[9]		50.00%		55.00%		45.90%	[10]			43.74%	[11]			50.50%	[12]	32.28%	[13]				35.50%		40.00%	[14]		50.00%	[15]
Area of property (sqft)	28,549,979								28,549,979			834,000		30,000		30,000	[1]		120,000	[2]	526,000		674,000		640,000		582,000		2,600,000	[3]	71,000	[4]	304,000		17,000	[5]	37,000			769,000	[6]		1,237,000	[7]		49,000	[8]	13,045		10,000	[9]		65,000		390,000		145,000	[10]		4,500,000	4,474,000	[11]			460,000	[12]	17,000	[13]				148,000		365,000	[14]		32,000	[15]
Acquisition Price												95,800,000	[16]	22,400,000	[16]	4,400,000	[1],[16]		251,900,000	[16],[2]	285,000,000	[16]	520,000,000	[16]	210,000,000	[16]	111,500,000	[16]	1,575,000,000	[16],[3]	43,600,000	[16],[4]	193,000,000	[16]	10,800,000	[16],[5]	4,000,000	[16]		500,000,000	[16],[6]		400,000,000	[16],[7]		136,550,000	[16],[8]			66,250,000	[16],[9]		223,000,000	[16]	252,500,000	[16]	31,000,000	[10],[16]			880,103,000	[11],[16]			315,000,000	[12],[16]	75,000,000	[13],[16]				45,000,000	[16]	50,000,000	[14],[16]		0	[15],[16]
Beneficial interest sold (as a percent)																		50.00%
Sales price																		617,600,000																																														315,000,000
Proceeds from sale of equity method investments																		67,900,000																																											224,300,000
Recognized gain (loss) on sale of interest in property	7,538,000	(354,000)	(3,583,000)	0	19,277,000	(4,807,000)	15,323,000	7,260,000	3,601,000	37,053,000	2,918,000							3,000,000																					0																						2,100,000				2,800,000																		19,400,000
Employee compensation award																		1,000,000																																														1,000,000
Valuation of investment under the recapitalization transaction																																										286,600,000			1,100,000,000
Effective ownership interest in underlying investment (as a percent)																																										50.00%
Additional income upon the contribution of debt investment to joint venture									193,843,000	119,155,000	120,418,000																															38,700,000
Debt investments contributed to joint venture									0	25,362,000	286,571,000																															111,300,000
Floating rate financing assumed by joint venture									0	0	30,000,000																															30,000,000
Number of units in property (units)																																																				3																	3	2	4
Number of units in real estate property acquired (units)																																																	2
Payments to acquire equity method investments									150,274,000	215,174,000	109,920,000																																						7,500,000																											51,000,000
Number of floors of student housing (floors)																																																									30
Number of properties (properties)																																																												31
Prior to the recapitalization positions held in mezzanine and preferred equity																																																												678,800,000
Period of extension option for mortgage secured by the portfolio																																																												1 year
Number of properties sold (properties)																																																													3
Repayments of loan																																																													194,500,000																					20,500,000		194,500,000
Remaining percentage of interest acquired in joint venture (percent)																																																											10.73%
Partnership interest sold to Harel by joint venture (as a percent)																																																											20.78%		6.00%																			49.50%
Term																																																												2 years																					7 years
Interest rate, description																																																																LIBOR
Interest rate added to base rate (as a percent)																		2.75%																																														2.20%
Ownership interest sold (as a percent)																																																											20.78%		6.00%																			49.50%
Face amount of loan																																																												68,000,000.0			26,700,000	150,000,000																		20,500,000
Mortgage on properties assumed																																																																																	$ 170,000,000
Interest rate (as a percent)																																																																										8.75%

[1] In March 2013, Sutton conveyed his interest in this property to us.

[2] In June 2012, this retail condominium was recapitalized. The recapitalization triggered a promote to our partner, which resulted in a reduction of our economic interest. In addition, we sold 50% of our remaining interest at a property valuation of $617.6 million. 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, which is net of a $1.0 million employee compensation award, accrued in connection with the realization of this investment gain as a bonus to certain employees that were instrumental in realizing the gain on this sale.

[3] The property is subject to a triple-net lease arrangement with a single tenant, which expires in 2020. In December 2013, the joint venture signed an agreement extending the lease through December 31, 2035. The agreement includes an option for the tenant to acquire the property for a specified price during the period from December 1, 2017 through December 31, 2020.

[4] In June 2013, the joint venture completed its redevelopment project. In July 2013, the lease for Pace University, or Pace, its primary tenant, commenced.

[5] The property is subject to a long-term net lease arrangement.

[6] 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, no gain was recognized on this sale.

[7] In March 2011, we contributed our debt investment with a carrying value of $286.6 million to a newly formed joint venture in which we hold a 50% interest. We realized $38.7 million of additional income upon the contribution. This income is included in investment income in the consolidated statements of income. The joint venture paid us approximately $111.3 million and also assumed $30.0 million of related floating rate financing which matures in June 2016. In May 2011, this joint venture took control of the underlying property as part of a recapitalization transaction which valued the investment at approximately $1.1 billion.

[8] In connection with this acquisition, the joint venture also acquired a long-term leasehold interest in the retail space and certain other spaces at 1560 Broadway, which is adjacent to 1552 Broadway. The purchase price relates only to the purchase of the 1552 Broadway interest which comprises 13,045 square feet. In 2012, we, along with Sutton, acquired the property at 155 West 46th Street, which is adjacent to 1552 and 1560 Broadway, and sold it to the fee owner of 1560 Broadway.

[9] The joint venture owns 100% interest as tenant-in-common in 30 East 65th Street Corporation and the related proprietary lease of three cooperative apartment units in the building. In October 2013, the joint ventured acquired two additional cooperative apartment units in the building for $7.5 million.

[10] The joint venture acquired the fee interest in the property and will develop an approximately 30 story building for student housing. Upon completion of the development, the joint venture will convey a long-term ground lease condominium interest in the building to Pace.

[11] In September 2012, the Company, together with an affiliate of Blackstone, Gramercy and Square Mile Capital Management LLC, or Square Mile, formed a joint venture to recapitalize a 31-property, 4.5-million-square-foot West Coast office portfolio. The joint venture extended the $678.8 million mortgage secured by the portfolio for a term of 2 years with a 1-year extension option. In addition, the joint venture entered into a new $68.0 million mezzanine loan for a term of 2 years. Prior to the recapitalization in September 2012, the Company held $26.7 million in mezzanine and preferred equity positions in the entity that owned the portfolio. Following the recapitalization, Blackstone became the majority owner of the joint venture, with Equity Office Properties, a Blackstone affiliate, being responsible for the portfolio’s management and leasing. In February 2013, we acquired Gramercy’s 10.73% interest in the joint venture and simultaneously sold 20.78% of the newly acquired interest to Square Mile Capital Management LLC or Square Mile. During the year ended December 31, 2013, we acquired Square Mile’s 6.00% interest in the joint venture and the joint venture sold three of the properties for an aggregate of $224.3 million, on which we recognized a gain of approximately $2.1 million. The proceeds from the sale of these properties were used primarily to repay $194.5 million of the mortgage and $20.5 million of the mezzanine loan.

[12] In November 2012, we sold our 49.5% partnership interest in 521 Fifth Avenue to Plaza Global Real Estate Partners for a gross valuation price of $315.0 million for this property. We recognized a gain of $19.4 million on the sale which is net of a $1.0 million employee compensation award, accrued in connection with the realization of this investment gain as a bonus to certain employees that were instrumental in realizing the gain on this sale. We also refinanced the existing $150.0 million loan with a $170.0 million 7-year mortgage loan which bears interest at 220 basis points over LIBOR. Following the sale, we deconsolidated the entity effective November 30, 2012 and have accounted our investment under the equity method because of lack of control. During the year ended December 31, 2013, we recognized additional post closing costs of $2.8 million as an adjustment to the gain.

[13] We hold a 32.28% interest in 3 retail and 2 residential units at the property and a 16.14% in 4 residential units at the property.

[14] The joint venture owned a preferred equity interest in an entity that holds the interest in a mixed commercial use property located in Manhattan. The preferred equity bore interest at a rate of 8.75% per annum through its redemption date in December 2013.

[15] 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 an 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. In November 2013, the joint venture signed an agreement to buy out the lease of retailer Juicy Couture for $51.0 million as part of its plan to redevelop and reposition the property. Under this agreement, the tenant shall terminate the lease no later than April 2014.

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