Mortgages and Other Loans Payable (Details)		12 Months Ended
	Dec. 06, 2015 USD ($)	Dec. 31, 2015 USD ($) advance $ / shares	Feb. 28, 2015	Jan. 31, 2015	Dec. 31, 2014 USD ($)	Oct. 31, 2014 USD ($)	May. 31, 2014 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt		$ 5,816,010,000			$ 4,736,709,000
Total floating rate debt		1,205,494,000			1,103,942,000
Total fixed rate and floating rate debt		7,021,504,000			5,840,651,000
Mortgages reclassed to liabilities related to assets held for sale		(29,000,000)			(253,942,000)
Total mortgages and other loans payable		6,992,504,000			5,586,709,000
Federal Home Loan Bank Advances outstanding		$ 45,800,000
Federal Home Loan Bank, advances, weighted-average borrowing rate (as a percent)		0.55%
Book value of collateral		$ 10,800,000,000			8,200,000,000
Series J Preferred Units
Debt Instrument [Line Items]
Total fixed rate debt		4,000,000			4,000,000
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest		$ 4,000,000
Preferred Units (as a percent)		3.75%
Preferred Units, liquidation preference (in dollars per unit) | $ / shares		$ 1,000.00
Series J Preferred Units | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.75%
Master Repurchase Agreement
Debt Instrument [Line Items]
Total floating rate debt		$ 253,424,000			100,000,000
Maximum facility capacity		$ 300,000,000.0
Basis Point Fee (as a percent)	0.25%
Threshold amount for basis point fee to be applicable (less than)	$ 150,000,000.0
Master Repurchase Agreement | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		3.36%
Master Repurchase Agreement | Minimum | LIBOR
Debt Instrument [Line Items]
Credit facility, interest rate (as a percent)		2.50%
Master Repurchase Agreement | Maximum | LIBOR
Debt Instrument [Line Items]
Credit facility, interest rate (as a percent)		3.25%
FHLB Facility
Debt Instrument [Line Items]
Total floating rate debt		$ 45,750,000			0
Number of Federal Home Loan Bank Advances | advance		4
FHLB Facility | Minimum
Debt Instrument [Line Items]
Federal Home Loan Bank, advances, weighted-average borrowing rate (as a percent)		0.50%
FHLB Facility | Maximum
Debt Instrument [Line Items]
Federal Home Loan Bank, advances, weighted-average borrowing rate (as a percent)		0.58%
FHLB Advance Maturing in April 2016
Debt Instrument [Line Items]
Federal Home Loan Bank Advances outstanding		$ 1,000,000
FHLB Advance Maturing in April 2016, 2
Debt Instrument [Line Items]
Federal Home Loan Bank Advances outstanding		15,800,000
FHLB Advance Maturing in June 2016
Debt Instrument [Line Items]
Federal Home Loan Bank Advances outstanding		5,000,000
FHLB Advance Maturing in December 2016
Debt Instrument [Line Items]
Federal Home Loan Bank Advances outstanding		24,000,000
500 West Putnam Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 22,376,000			22,968,000
500 West Putnam Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		5.52%
Landmark Square
Debt Instrument [Line Items]
Total fixed rate debt		$ 79,562,000			81,269,000
Landmark Square | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.00%
485 Lexington Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 450,000,000			450,000,000
485 Lexington Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		5.61%
762 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 7,872,000			8,045,000
762 Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.84%	3.86%	3.75%
885 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 267,650,000			267,650,000
885 Third Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		6.26%
1745 Broadway
Debt Instrument [Line Items]
Total fixed rate debt		$ 16,000,000			16,000,000
1745 Broadway | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.81%
388-390 Greenwich Street
Debt Instrument [Line Items]
Total fixed rate debt		$ 1,004,000,000			1,004,000,000
Total floating rate debt		$ 446,000,000			446,000,000
388-390 Greenwich Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.25%
Interest rate, floating rate debt (as a percent)		1.94%
388-390 Greenwich Street | Swap One
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)							3.80%
Total fixed rate debt							$ 504,000,000
388-390 Greenwich Street | Swap Two
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)						2.69%
Total fixed rate debt						$ 500,000,000
One Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 542,817,000			565,742,000
One Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		5.91%
100 Church Street
Debt Instrument [Line Items]
Total fixed rate debt		$ 225,099,000			228,612,000
100 Church Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.68%
919 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 500,000,000			500,000,000
Interest in property (as a percent)		51.00%
919 Third Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		5.12%
400 East 57th Street
Debt Instrument [Line Items]
Total fixed rate debt		$ 67,644,000			68,896,000
400 East 57th Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.13%
400 East 58th Street
Debt Instrument [Line Items]
Total fixed rate debt		$ 28,990,000			29,527,000
400 East 58th Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.13%
420 Lexington Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 300,000,000			300,000,000
420 Lexington Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.99%
1515 Broadway
Debt Instrument [Line Items]
Total fixed rate debt		$ 900,000,000			900,000,000
1515 Broadway | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.93%
11 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 1,400,000,000			0
11 Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.84%
711 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 0			120,000,000
120 West 45th Street
Debt Instrument [Line Items]
Total fixed rate debt		0			170,000,000
600 Lexington Avenue
Debt Instrument [Line Items]
Total floating rate debt		$ 112,795,000			0
600 Lexington Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.30%
187 Broadway & 5-7 Dey Street
Debt Instrument [Line Items]
Total floating rate debt		$ 40,000,000			0
187 Broadway & 5-7 Dey Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.85%
1080 Amsterdam
Debt Instrument [Line Items]
Total floating rate debt		$ 3,525,000			0
1080 Amsterdam | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		3.96%
248-252 Bedford Avenue
Debt Instrument [Line Items]
Total floating rate debt		$ 29,000,000			29,000,000
248-252 Bedford Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		1.69%
220 East 42nd Street
Debt Instrument [Line Items]
Total floating rate debt		$ 275,000,000			275,000,000
220 East 42nd Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		1.80%
180 Maiden Lane
Debt Instrument [Line Items]
Total floating rate debt		$ 0			$ 253,942,000