Fair Value of Financial Instruments (Financial Instruments Not Carried at Fair Value) (Details) - Mortgages Note Payable - Significant Unobservable Inputs - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
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	153,182	142,874
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	104,282	97,448
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	54,687	50,424
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	11,136	10,446
9 Times Square | Gross Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	55,000	0
9 Times Square | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	54,712	0
196 Orchard Street | Gross Principal Balance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	51,000	0
196 Orchard Street | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	$ 52,717	$ 0