Investments in Unconsolidated Joint Ventures (Details) $ in Thousands	1 Months Ended				3 Months Ended	12 Months Ended
	Nov. 30, 2016 ft²	Oct. 31, 2016	Aug. 31, 2016 ft²	Oct. 31, 2015	Mar. 31, 2016	Dec. 31, 2016 USD ($) ft² unit	Sep. 30, 2016	Apr. 30, 2016	Dec. 31, 2015 USD ($)	Jul. 31, 2014 USD ($)
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary | $						$ 220,100			$ 39,700
Approximate Square Feet (sqft)						32,922,082
Jericho Plaza
General information on each joint venture
Ownership percentage in disposed asset					66.11%
11 Madison Avenue
General information on each joint venture
Approximate Square Feet (sqft)			2,314,000
Ownership percentage in disposed asset			40.00%
400 East 57th Street
General information on each joint venture
Approximate Square Feet (sqft)			290,482
Ownership percentage in disposed asset		49.00%
100 Park Avenue
General information on each joint venture
Ownership Interest						49.90%
Economic Interest (as a percent)						49.90%
Approximate Square Feet (sqft)						834,000
Acquisition Price | $						$ 95,800
717 Fifth Avenue
General information on each joint venture
Ownership Interest						10.92%
Economic Interest (as a percent)						10.92%
Approximate Square Feet (sqft)						119,500
Acquisition Price | $						$ 251,900
800 Third Avenue
General information on each joint venture
Ownership Interest						60.52%
Economic Interest (as a percent)						60.52%
Approximate Square Feet (sqft)						526,000
Acquisition Price | $						$ 285,000
1745 Broadway
General information on each joint venture
Ownership Interest						56.88%
Economic Interest (as a percent)						56.88%
Approximate Square Feet (sqft)						674,000
Acquisition Price | $						$ 520,000
Jericho Plaza
General information on each joint venture
Ownership Interest						11.67%	20.26%
Economic Interest (as a percent)						11.67%
Approximate Square Feet (sqft)						640,000
Acquisition Price | $						$ 210,000
Ownership interest acquired (percent)				57.52%
11 West 34th Street
General information on each joint venture
Ownership Interest						30.00%
Economic Interest (as a percent)						30.00%
Approximate Square Feet (sqft)						17,150
Acquisition Price | $						$ 10,800
3 Columbus Circle
General information on each joint venture
Ownership Interest						48.90%
Economic Interest (as a percent)						48.90%
Approximate Square Feet (sqft)						741,500
Acquisition Price | $						$ 500,000
280 Park Avenue
General information on each joint venture
Ownership Interest						50.00%
Economic Interest (as a percent)						50.00%
Approximate Square Feet (sqft)						1,219,158
Acquisition Price | $						$ 400,000
1552-1560 Broadway
General information on each joint venture
Ownership Interest						50.00%
Economic Interest (as a percent)						50.00%
Approximate Square Feet (sqft)						35,897
Acquisition Price | $						$ 136,550
724 Fifth Avenue
General information on each joint venture
Ownership Interest						50.00%
Economic Interest (as a percent)						50.00%
Approximate Square Feet (sqft)						65,040
Acquisition Price | $						$ 223,000
10 East 53rd Street
General information on each joint venture
Ownership Interest						55.00%
Economic Interest (as a percent)						55.00%
Approximate Square Feet (sqft)						354,300
Acquisition Price | $						$ 252,500
521 Fifth Avenue
General information on each joint venture
Ownership Interest						50.50%
Economic Interest (as a percent)						50.50%
Approximate Square Feet (sqft)						460,000
Acquisition Price | $						$ 315,000
21 East 66th Street
General information on each joint venture
Ownership Interest						32.28%
Economic Interest (as a percent)						32.28%
Approximate Square Feet (sqft)						16,736
Acquisition Price | $						$ 75,000
650 Fifth Avenue
General information on each joint venture
Ownership Interest						50.00%
Economic Interest (as a percent)						50.00%
Approximate Square Feet (sqft)						69,214
Acquisition Price | $						$ 0
Additional square feet acquired	36,890
Lease term	16 years 10 months
121 Greene Street
General information on each joint venture
Ownership Interest						50.00%
Economic Interest (as a percent)						50.00%
Approximate Square Feet (sqft)						7,131
Acquisition Price | $						$ 27,400
175-225 Third Street Brooklyn, New York
General information on each joint venture
Ownership Interest						95.00%
Economic Interest (as a percent)						95.00%
Approximate Square Feet (sqft)						0
Acquisition Price | $						$ 74,600
55 West 46th Street
General information on each joint venture
Ownership Interest						25.00%
Economic Interest (as a percent)						25.00%
Approximate Square Feet (sqft)						347,000
Acquisition Price | $						$ 295,000
Stonehenge Portfolio
General information on each joint venture
Ownership Interest		90.00%
Approximate Square Feet (sqft)						2,046,733
Acquisition Price | $						$ 36,668
131-137 Spring Street
General information on each joint venture
Ownership Interest						20.00%
Economic Interest (as a percent)						20.00%
Approximate Square Feet (sqft)						68,342
Acquisition Price | $						$ 277,750
76 11th Avenue
General information on each joint venture
Ownership Interest						33.33%
Economic Interest (as a percent)						35.09%
Approximate Square Feet (sqft)						764,000
Acquisition Price | $						$ 138,240
Number of Mezzanine Notes Secured | unit						2
Exit Fee, Percentage Owned						50.00%
Extension Fee, Percentage Owned						38.00%
605 West 42nd Street
General information on each joint venture
Ownership Interest						20.00%
Economic Interest (as a percent)						20.00%
Approximate Square Feet (sqft)						927,358
Acquisition Price | $						$ 759,000
11 Madison Avenue
General information on each joint venture
Ownership Interest						60.00%
Economic Interest (as a percent)						60.00%
Approximate Square Feet (sqft)						2,314,000
Acquisition Price | $						$ 2,605,000
333 East 22nd St
General information on each joint venture
Ownership Interest						33.33%
Economic Interest (as a percent)						33.33%
Approximate Square Feet (sqft)						26,926
Acquisition Price | $						$ 0
400 East 57th Street
General information on each joint venture
Ownership Interest						51.00%
Economic Interest (as a percent)						41.00%
Approximate Square Feet (sqft)						290,482
Acquisition Price | $						$ 170,000
Ownership percentage in disposed asset		49.00%
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft)						13,045
Joint venture | Stonehenge Portfolio
General information on each joint venture
Ownership Interest		10.00%
Joint venture | Partners' Interest | 76 11th Avenue
General information on each joint venture
Exit Fee, Percentage Owned						25.00%
Extension Fee, Percentage Owned						31.00%
Joint venture | Partners' Interest | 11 Madison Avenue
General information on each joint venture
Equity method investments, depreciation | $						$ 8,400
Mezzanine Loan Repaid in April 2016
General information on each joint venture
Ownership Interest								20.00%
Loans and Leases Receivable, Gross | $										$ 50,000
21 East 66th Street | Three Retail and Two Residential Units
General information on each joint venture
Ownership Interest						32.28%
Number of Stores | unit						3
Number Of Residential Units | unit						2
21 East 66th Street | Three Residential Units
General information on each joint venture
Ownership Interest						16.14%
Number Of Residential Units | unit						3