Investment in Unconsolidated Joint Ventures (Details 2) (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	Sep. 30, 2014		May 31, 2014		Mar. 31, 2014		Dec. 31, 2013		Jan. 31, 2014 sqft		Jul. 31, 2012		Apr. 30, 2012		Mar. 31, 2012		Oct. 01, 2012	Sep. 30, 2012
Investments in Unconsolidated Joint Ventures [Abstract]
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
Total floating rate debt	1,103,942,000						729,949,000					1,103,942,000	729,949,000									729,949,000
Approximate Square Feet (sqft)	33,891,036											33,891,036
Mortgage assigned to joint venture												150,000,000	0		0
180 Broadway
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage		25.50%	[1]													25.50%	[1]
Implied Sales Price		222,500,000	[1],[2]													222,500,000	[1],[2]
Gain on Sale																16,500,000	[1],[3]
Promote recognized																3,300,000
747 Madison Avenue
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage																		33.33%	[4]
Implied Sales Price																		160,000,000	[2],[4]
Gain on Sale																		0	[3],[4]
Promote recognized																		10,300,000
Preferred equity investment																		30,000,000
Deferred gain on sale																		13,100,000
West Coast office portfolio
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage					42.02%	[5]	42.04%	[5]					42.04%	[5]						42.02%	[5]	42.04%	[5]									27.63%	63.18%
Implied Sales Price					756,000,000	[2],[5]	224,300,000	[2],[5]					224,300,000	[2],[5]						756,000,000	[2],[5]	224,300,000	[2],[5]
Gain on Sale																				85,600,000	[3],[5]	2,100,000	[3],[5]
Economic interest							43.74%						43.74%									43.74%
West Coast Office Portfolio, Two Joint Venture Partners
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage							14.39%						14.39%									14.39%
21-25 West 34th Street
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage																								49.90%	[6]
Implied Sales Price																								114,900,000	[2],[6]
Gain on Sale																								20,900,000	[3],[6]
Approximate Square Feet (sqft)																								91,300
27-29 West 34th Street
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage							50.00%	[7]					50.00%	[7]								50.00%	[7]
Implied Sales Price							70,100,000	[2],[7]					70,100,000	[2],[7]								70,100,000	[2],[7]
Gain on Sale																						7,600,000	[3],[7]
Accrued employee compensation award							1,500,000						1,500,000									1,500,000
One Court Square
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage																										30.00%	[8]
Implied Sales Price																										481,100,000	[2],[8]
Gain on Sale																										1,000,000	[3],[8]
Mortgage assigned to joint venture																										315,000,000
379 West Broadway
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage																												45.00%	[9]
Implied Sales Price																												48,500,000	[2],[9]
Gain on Sale																												6,500,000	[3],[9]
Fees Position Acquired																												13,500,000
141 Fifth Avenue
Investments in Unconsolidated Joint Ventures [Abstract]
Ownership Percentage																														50.00%	[10]
Implied Sales Price																														46,000,000	[10],[2]
Gain on Sale																														7,300,000	[10],[3]
Employee compensation award																														1,500,000
Joint venture
Investment in Unconsolidated Joint Ventures
Maximum amount of loan recourse to entity	18,400,000											18,400,000
Investments in Unconsolidated Joint Ventures [Abstract]
Total floating rate debt	$ 1,713,440,000						$ 1,842,815,000					$ 1,713,440,000	$ 1,842,815,000									$ 1,842,815,000
Preferred equity | Herald Center
Investments in Unconsolidated Joint Ventures [Abstract]
Interest rate (as a percent)							8.75%						8.75%									8.75%

[1] In connection with the sale of the property, we also recognized a promote of $3.3 million.

[2] Represents implied gross valuation for the joint venture or sales price of the property.

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

[4] We sold our ownership interest in the joint venture, which owns 100% interest as tenant-in-common in 30 East 65th Street Corporation and the related proprietary lease of five cooperative apartment units in the property. We also recognized a promote of $10.3 million and originated a $30.0 million preferred equity investment. Given our continuing involvement as a preferred equity holder, we deferred the gain on sale of $13.1 million as we did not meet the requirements of a sale under the full accrual method. We, along with our joint venture partners, retained one apartment unit at this property.

[5] 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.

[6] We sold our ownership interest in the joint venture. We, along with our joint venture partner, retained approximately 91,300 square feet (unaudited) of development rights at the property.

[7] We sold our ownership interest in the joint venture. The gain on the sale of this investment is net of a $1.5 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. Simultaneously, we, along with Sutton, also formed a new joint venture and retained the air rights at this property.

[8] We, along with our joint venture partner, sold the property, which included the assumption by the purchaser of $315.0 million of existing debt.

[9] We, along with our joint venture partner, sold the property, inclusive of the fee position, which was acquired for $13.5 million.

[10] We, along with our joint venture partner, sold the property. The gain on the sale of this investment is net of a $1.5 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.