Investments in Unconsolidated Joint Ventures - Mortgages and Other Loans Payable (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) extension	Dec. 31, 2017 USD ($)	Oct. 31, 2016
Debt Instrument [Line Items]
Long-term debt	$ 1,961,240,000	$ 2,837,282,000
Total fixed rate debt	1,389,717,000	2,400,560,000
Total floating rate debt	598,442,000	465,431,000
Total fixed rate and floating rate debt	1,988,159,000	2,865,991,000
Deferred financing costs, net	(26,919,000)	(28,709,000)
919 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt	$ 0	500,000,000
400 East 57th Street
Debt Instrument [Line Items]
Ownership Interest Sold	51.00%
Stonehenge Portfolio
Debt Instrument [Line Items]
Ownership Interest Sold			90.00%
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt	$ 5,666,108,000	5,569,138,000
Total floating rate debt	3,387,705,000	3,979,066,000
Total fixed rate and floating rate debt	9,053,813,000	9,548,204,000
Deferred financing costs, net	(103,191,000)	(136,103,000)
Total joint venture mortgages and other loans payable, net	8,950,622,000	9,412,101,000
Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Total floating rate debt	$ 0	345,000,000
Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest, percent	50.50%
Interest rate, fixed rate debt (as a percent)	3.73%
Total fixed rate debt	$ 170,000,000	170,000,000
Ownership Interest Sold	50.50%
Joint venture | 717 Fifth Avenue
Debt Instrument [Line Items]
Ownership Interest Sold	10.92%
Joint venture | 717 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.45%
Total fixed rate debt	$ 300,000,000	300,000,000
Committed amount	$ 300,000,000
Joint venture | 717 Fifth Avenue | Mezzanine loans
Debt Instrument [Line Items]
Economic Interest, percent	10.92%
Interest rate, fixed rate debt (as a percent)	5.50%
Total fixed rate debt	$ 355,328,000	355,328,000
Committed amount	$ 355,300,000
Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Economic Interest, percent	32.28%
Interest rate, fixed rate debt (as a percent)	3.60%
Total fixed rate debt	$ 12,000,000	12,000,000
Total floating rate debt	$ 1,571,000	1,648,000
Ownership Interest Sold	32.28%
Joint venture | 919 Third Avenue
Debt Instrument [Line Items]
Economic Interest, percent	51.00%
Interest rate, fixed rate debt (as a percent)	5.12%
Total fixed rate debt	$ 500,000,000	0
Ownership Interest Sold	51.00%
Joint venture | 1515 Broadway
Debt Instrument [Line Items]
Economic Interest, percent	56.87%
Interest rate, fixed rate debt (as a percent)	3.93%
Total fixed rate debt	$ 855,876,000	872,528,000
Ownership Interest Sold	56.87%
Joint venture | 11 Madison Avenue
Debt Instrument [Line Items]
Economic Interest, percent	60.00%
Interest rate, fixed rate debt (as a percent)	3.84%
Total fixed rate debt	$ 1,400,000,000	1,400,000,000
Ownership Interest Sold	60.00%
Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Economic Interest, percent	60.52%
Interest rate, fixed rate debt (as a percent)	3.37%
Total fixed rate debt	$ 177,000,000	177,000,000
Ownership Interest Sold	60.52%
Joint venture | 400 East 57th Street
Debt Instrument [Line Items]
Economic Interest, percent	41.00%
Interest rate, fixed rate debt (as a percent)	3.00%
Total fixed rate debt	$ 99,828,000	100,000,000
Ownership Interest Sold	51.00%
Joint venture | Worldwide Plaza
Debt Instrument [Line Items]
Economic Interest, percent	24.35%
Interest rate, fixed rate debt (as a percent)	3.98%
Total fixed rate debt	$ 1,200,000,000	1,200,000,000
Ownership Interest Sold	24.35%
Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.20%
Total fixed rate debt	$ 321,076,000	357,282,000
Ownership Interest Sold			10.00%
Joint venture | Stonehenge Portfolio | Initial Maturity August 2019
Debt Instrument [Line Items]
Total fixed rate debt	134,300,000
Joint venture | Stonehenge Portfolio | Initial Maturity June 2024
Debt Instrument [Line Items]
Total fixed rate debt	54,100,000
Joint venture | Stonehenge Portfolio | Secured Debt, Initial Maturity April 2028
Debt Instrument [Line Items]
Total fixed rate debt	132,600,000
Joint venture | 3 Columbus Circle
Debt Instrument [Line Items]
Total fixed rate debt	0	350,000,000
Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Total floating rate debt	$ 0	81,099,000
Joint venture | 280 Park Avenue
Debt Instrument [Line Items]
Economic Interest, percent	50.00%
Interest rate, floating rate debt (as a percent)	1.73%
Total floating rate debt	$ 1,200,000,000	1,200,000,000
Ownership Interest Sold	50.00%
Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Economic Interest, percent	55.00%
Interest rate, floating rate debt (as a percent)	2.25%
Total floating rate debt	$ 170,000,000	170,000,000
Ownership Interest Sold	55.00%
Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt	$ 0	275,000,000
Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Economic Interest, percent	25.00%
Interest rate, floating rate debt (as a percent)	2.13%
Total floating rate debt	$ 185,569,000	171,444,000
Committed amount	195,000,000
Unused borrowing capacity, amount	$ 9,400,000
Ownership Interest Sold	25.00%
Joint venture | 175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Total floating rate debt	$ 0	40,000,000
Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Economic Interest, percent	20.00%
Total floating rate debt	$ 550,000,000	550,000,000
Ownership Interest Sold	20.00%
Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Ownership Interest Sold	50.00%
Joint venture | 650 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Economic Interest, percent	50.00%
Interest rate, fixed rate debt (as a percent)	4.46%
Long-term debt	$ 210,000,000
Total fixed rate debt	$ 210,000,000	210,000,000
Joint venture | 650 Fifth Avenue | Mezzanine loans
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	5.45%
Total fixed rate debt	$ 65,000,000	65,000,000
Joint venture | 650 Fifth Avenue | Mezzanine Loan, Initial Maturity October 2022 | Mezzanine loans
Debt Instrument [Line Items]
Long-term debt	$ 65,000,000
Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Economic Interest, percent	50.00%
Interest rate, floating rate debt (as a percent)	1.50%
Total floating rate debt	$ 15,000,000	15,000,000
Ownership Interest Sold	50.00%
Joint venture | 131-137 Spring Street
Debt Instrument [Line Items]
Economic Interest, percent	20.00%
Interest rate, floating rate debt (as a percent)	1.55%
Total floating rate debt	$ 141,000,000	141,000,000
Ownership Interest Sold	20.00%
Joint venture | 1552-1560 Broadway
Debt Instrument [Line Items]
Economic Interest, percent	50.00%
Interest rate, floating rate debt (as a percent)	2.65%
Total floating rate debt	$ 195,000,000	195,000,000
Ownership Interest Sold	50.00%
Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Economic Interest, percent	30.00%
Interest rate, floating rate debt (as a percent)	1.45%
Total floating rate debt	$ 23,000,000	23,000,000
Ownership Interest Sold	30.00%
Joint venture | 103 East 86th Street
Debt Instrument [Line Items]
Economic Interest, percent	1.00%
Interest rate, floating rate debt (as a percent)	1.40%
Total floating rate debt	$ 38,000,000	55,340,000
Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Economic Interest, percent	49.90%
Interest rate, floating rate debt (as a percent)	1.75%
Total floating rate debt	$ 360,000,000	360,000,000
Ownership Interest Sold	49.90%
Joint venture | One Vanderbilt
Debt Instrument [Line Items]
Economic Interest, percent	71.01%
Interest rate, floating rate debt (as a percent)	2.75%
Total floating rate debt	$ 375,000,000	355,535,000
Ownership Interest Sold	71.01%
Joint venture | One Vanderbilt | Construction Loans
Debt Instrument [Line Items]
Maximum facility capacity	$ 1,750,000,000.00
Initial Term (in Years)	5 years
Number of extension options | extension	2
Period of extension option	1 year
Joint venture | 2 Herald Square
Debt Instrument [Line Items]
Economic Interest, percent	51.00%
Interest rate, floating rate debt (as a percent)	1.55%
Total floating rate debt	$ 133,565,000	$ 0
Committed amount	$ 150,000,000
Ownership Interest Sold	51.00%
Joint venture | Weighted Average | 21 East 66th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.75%
Joint venture | Weighted Average | Stonehenge Portfolio
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)
Joint venture | Weighted Average | 605 West 42nd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	1.44%