Fair Value of Financial Instruments (Financial Instruments Not Carried at Fair Value) (Details) - Mortgages Note Payable - Significant Unobservable Inputs - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
123 William Street | Gross Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	$ 140,000	$ 140,000
123 William Street | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	150,224	151,428
1140 Avenue of the Americas | Gross Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	99,000	99,000
1140 Avenue of the Americas | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	102,740	103,340
400 E. 67th Street - Laurel Condominium / 200 Riverside Boulevard - ICON Garage | Gross Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	50,000	50,000
400 E. 67th Street - Laurel Condominium / 200 Riverside Boulevard - ICON Garage | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	53,406	53,951
8713 Fifth Avenue | Gross Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	10,000	10,000
8713 Fifth Avenue | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	11,044	11,175
9 Times Square | Gross Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	55,000	55,000
9 Times Square | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	52,058	54,759
196 Orchard Street | Gross Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	51,000	51,000
196 Orchard Street | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	$ 51,748	$ 52,369