Properties Held for Sale and Property Dispositions (Properties Held for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Depreciable Real Estate Reserves, Effect on Earnings, Attributable to Parent	$ (53,808)	$ 0
Greene Street 102 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Joint Venture Ownership Percentage Sold	90.00%
White Plains Road, 520 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Depreciable Real Estate Reserves, Effect on Earnings, Attributable to Parent	$ 14,200
One Vanderbilt
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Joint Venture Ownership Percentage Sold	30.00%