Fair Value of Financial Instruments (Financial Instruments Not Carried at Fair Value) (Details) - Mortgages note payable - Significant unobservable inputs - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	144,281	147,531
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	$ 98,006		$ 100,036