Significant Accounting Policies (Concentrations of Credit Risk/Accounting Standards Updates)(Details) - Annualized rent - Customer concentration	3 Months Ended
	Mar. 31, 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%
1515 Broadway
Concentration of Credit Risk
Percentage of concentration (percent)	8.10%
919 Third Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	8.20%
1185 Avenue of the Americas
Concentration of Credit Risk
Percentage of concentration (percent)	6.30%
420 Lexington Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	5.40%
1 Madison Ave
Concentration of Credit Risk
Percentage of concentration (percent)	5.10%
Tenant 1
Concentration of Credit Risk
Percentage of concentration (percent)	8.10%
Tenant 2
Concentration of Credit Risk
Percentage of concentration (percent)	6.70%