Debt - Summary of Outstanding Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Maturity date of debt	2017-10
Notes and Mortgages payable, Interest Rate	4.68%
Mortgages and notes payable	$ 2,941,524	$ 2,852,287
Credit Facility	$ 20,000
Credit Facility With Variable Rate [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maturity date of debt	2018-11
Interest Rate	1.54%
Credit Facility	$ 20,000
1633 Broadway [Member]
Debt Instrument [Line Items]
Notes and Mortgages payable, Interest Rate	3.52%
Mortgages and notes payable	$ 1,013,544	926,260
1633 Broadway [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2022-12
Notes and Mortgages payable, Interest Rate	3.54%
Mortgages and notes payable	$ 1,000,000	772,100
1633 Broadway [Member] | Mortgages and Notes Payable with Variable Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2022-12
Notes and Mortgages payable, Interest Rate	2.15%
Mortgages and notes payable	$ 13,544	154,160
900 Third Avenue [Member]
Debt Instrument [Line Items]
Notes and Mortgages payable, Interest Rate	4.22%
Mortgages and notes payable	$ 274,337	274,337
900 Third Avenue [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2017-11
Notes and Mortgages payable, Interest Rate	5.98%
Mortgages and notes payable	$ 162,000	255,000
900 Third Avenue [Member] | Mortgages and Notes Payable with Variable Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2017-11
Notes and Mortgages payable, Interest Rate	1.69%
Mortgages and notes payable	$ 112,337	19,337
31 West 52nd Street [Member]
Debt Instrument [Line Items]
Notes and Mortgages payable, Interest Rate	4.23%
Mortgages and notes payable	$ 413,490	413,490
31 West 52nd Street [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2017-12
Notes and Mortgages payable, Interest Rate	6.04%
Mortgages and notes payable	$ 237,600	337,500
31 West 52nd Street [Member] | Mortgages and Notes Payable with Variable Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2017-12
Notes and Mortgages payable, Interest Rate	1.79%
Mortgages and notes payable	$ 175,890	75,990
One Market Plaza [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2019-12
Notes and Mortgages payable, Interest Rate	6.14%
Mortgages and notes payable	$ 857,037	853,711
Waterview [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2017-06
Notes and Mortgages payable, Interest Rate	5.76%
Mortgages and notes payable	$ 210,000	210,000
1899 Pennsylvania Avenue [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2020-11
Notes and Mortgages payable, Interest Rate	4.88%
Mortgages and notes payable	$ 89,116	90,489
Liberty Place [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity date of debt	2018-06
Notes and Mortgages payable, Interest Rate	4.50%
Mortgages and notes payable	$ 84,000	$ 84,000