Significant Accounting Policies - Concentrations of Credit Risk/Accounting Standards Updates (Details) - Annualized rent - Customer concentration	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017 Tenant
Concentration of Credit Risk
Number of tenants (tenants)		2
Maximum percentage of annualized rent for any one tenant not individually disclosed (percent) (more than)	5.00%	5.00%
1515 Broadway
Concentration of Credit Risk
Percentage of concentration (percent)	8.90%
11 Madison Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	6.30%
1185 Avenue of the Americas
Concentration of Credit Risk
Percentage of concentration (percent)	6.20%
420 Lexington Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	5.40%
Tenant 1
Concentration of Credit Risk
Percentage of concentration (percent)		8.80%
Tenant 2
Concentration of Credit Risk
Percentage of concentration (percent)		6.70%