Debt - Mortgage Debt (Details) - Mortgages - USD ($)	Sep. 30, 2017	Jun. 30, 2017	May 31, 2017	Apr. 30, 2017
1542 Third Avenue
Debt Instrument [Line Items]
Face amount				$ 30,000,000.0
Stated rate (as a percent)	4.29%		4.23%	4.29%
10 Bank Street
Debt Instrument [Line Items]
Face amount			$ 35,000,000.0
Stated rate (as a percent)	4.23%
First Stamford Place
Debt Instrument [Line Items]
Face amount		$ 180,000,000.0
Stated rate (as a percent)	4.28%	4.28%
First Stamford Place - First Lien
Debt Instrument [Line Items]
Face amount	$ 164,000,000	$ 164,000,000.0
Stated rate (as a percent)	4.09%	4.09%
First Stamford Place - Second Lien
Debt Instrument [Line Items]
Face amount	$ 16,000,000	$ 16,000,000.0
Stated rate (as a percent)	6.25%	6.25%
1010 Third Avenue and 77 West 55th Street
Debt Instrument [Line Items]
Face amount		$ 40,000,000.0
Stated rate (as a percent)	4.01%	4.01%
383 Main Avenue
Debt Instrument [Line Items]
Face amount		$ 30,000,000.0
Stated rate (as a percent)	4.44%	4.44%