Debt - Summary of Outstanding Debt (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Maturity Date	2017-10
Interest Rate	5.51%
Mortgages and notes payable	$ 2,853,237	$ 2,852,287
1633 Broadway [Member]
Debt Instrument [Line Items]
Interest Rate	5.39%
Mortgages and notes payable	$ 926,260	926,260
1633 Broadway [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity Date	2016-12
Interest Rate	6.15%
Mortgages and notes payable	$ 772,100	772,100
1633 Broadway [Member] | Mortgages and Notes Payable with Variable Rate [Member]
Debt Instrument [Line Items]
Maturity Date	2016-12
Interest Rate	1.61%
Mortgages and notes payable	$ 154,160	154,160
900 Third Avenue [Member]
Debt Instrument [Line Items]
Interest Rate	5.10%
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	2017-11
Interest Rate	5.35%
Mortgages and notes payable	$ 255,000	255,000
900 Third Avenue [Member] | Mortgages and Notes Payable with Variable Rate [Member]
Debt Instrument [Line Items]
Maturity Date	2017-11
Interest Rate	1.77%
Mortgages and notes payable	$ 19,337	19,337
31 West 52nd Street [Member]
Debt Instrument [Line Items]
Interest Rate	4.99%
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	2017-12
Interest Rate	5.74%
Mortgages and notes payable	$ 337,500	337,500
31 West 52nd Street [Member] | Mortgages and Notes Payable with Variable Rate [Member]
Debt Instrument [Line Items]
Maturity Date	2017-12
Interest Rate	1.66%
Mortgages and notes payable	$ 75,990	75,990
One Market Plaza [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity Date	2019-12
Interest Rate	6.14%
Mortgages and notes payable	$ 855,339	853,711
Waterview [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity Date	2017-06
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	2020-11
Interest Rate	4.88%
Mortgages and notes payable	$ 89,811	90,489
Liberty Place [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Debt Instrument [Line Items]
Maturity Date	2018-06
Interest Rate	4.50%
Mortgages and notes payable	$ 84,000	$ 84,000