Significant Accounting Policies - Concentrations of Credit Risk/Accounting Standards Updates (Details) - Annualized rent - Customer concentration	9 Months Ended
	Sep. 30, 2020 Tenant
Concentration of Credit Risk
Number of tenants (tenants)	1
Maximum percentage of annualized rent for any one tenant not individually disclosed (percent) (more than)	5.00%
1185 Avenue of the Americas
Concentration of Credit Risk
Percentage of concentration (percent)	8.40%
11 Madison Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	8.10%
420 Lexington Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	7.40%
1515 Broadway
Concentration of Credit Risk
Percentage of concentration (percent)	6.30%
220 East 42nd Street
Concentration of Credit Risk
Percentage of concentration (percent)	5.90%
280 Park Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	5.40%
485 Lexington Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	5.00%
Tenant 1
Concentration of Credit Risk
Percentage of concentration (percent)	5.50%