Property Dispositions (Details) $ in Thousands	1 Months Ended				3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) ft²	Aug. 31, 2015 USD ($) ft²	May. 31, 2014 USD ($) ft²	Aug. 31, 2013 USD ($) ft²	Dec. 31, 2015 USD ($) ft²	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Usable Square Feet (sqft) | ft²	12,576,446				12,576,446								12,576,446
Gain (loss) on Sale									$ 0	$ (250)	$ 117,829	$ 0	$ 0	$ 117,579	$ 13,756
Impairment charge					$ 0	$ 9,998	$ 0	$ 0					9,998	0	0
Revenues
Rental revenue														7,853	28,843
Escalation and reimbursement revenues														1,080	4,093
Other income														0	8
Total revenues														8,933	32,944
Operating expenses														1,222	6,733
Real estate taxes														1,402	4,571
Ground rent														3,001	7,974
Interest expense, net of interest income														879	2,933
Depreciation and amortization														433	4,826
Total expenses														6,937	27,037
Net income from discontinued operations													$ 0	$ 1,996	$ 5,907
140 Grand Street
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Usable Square Feet (sqft) | ft²	130,100				130,100								130,100
Sales Price	$ 22,400
Gain (loss) on Sale	$ (10,500)
Impairment charge													$ 10,000
131-137 Spring Street
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Usable Square Feet (sqft) | ft²		68,342
Sales Price		$ 277,800
Gain (loss) on Sale		101,100
Employee compensation awards accrued		$ 4,100
Ownership percentage in disposed asset		80.00%
673 First Avenue
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Usable Square Feet (sqft) | ft²			422,000
Sales Price			$ 145,000
Gain (loss) on Sale			117,600
Employee compensation awards accrued			$ 3,400
333 West 34th Street
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Usable Square Feet (sqft) | ft²				345,400
Sales Price				$ 220,300
Gain (loss) on Sale				13,800
Employee compensation awards accrued				$ 3,000