Significant Accounting Policies (Concentrations of Credit Risk/Accounting Standards Updates)(Details) - Annualized rent - Customer concentration	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 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)	8.50%
1185 Avenue of the Americas
Concentration of Credit Risk
Percentage of concentration (percent)	6.30%
11 Madison Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	5.80%
420 Lexington Avenue
Concentration of Credit Risk
Percentage of concentration (percent)	5.60%
1 Madison Ave
Concentration of Credit Risk
Percentage of concentration (percent)	5.10%
Tenant 1
Concentration of Credit Risk
Percentage of concentration (percent)		8.40%
Tenant 2
Concentration of Credit Risk
Percentage of concentration (percent)		6.70%