Significant Accounting Policies (Concentrations of Credit Risk and Reclassification)(Details 3) - property	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Concentrations of Credit Risk
Number of Properties Transferred by Parent Company	2	5
Number of Entities Transferred by Parent Company	1
Annualized cash rent | Customer concentration
Concentrations of Credit Risk
Maximum percentage of annualized cash rent for any one tenant not individually disclosed (more than)	5.00%
Annualized cash rent | Customer concentration | 1185 Avenue of the Americas
Concentrations of Credit Risk
Percentage of concentration	15.40%
Annualized cash rent | Customer concentration | 625 Madison Avenue
Concentrations of Credit Risk
Percentage of concentration	9.70%
Annualized cash rent | Customer concentration | 919 Third Avenue
Concentrations of Credit Risk
Percentage of concentration	8.20%
Annualized cash rent | Customer concentration | 750 Third Avenue
Concentrations of Credit Risk
Percentage of concentration	7.70%
Annualized cash rent | Customer concentration | 810 Seventh Avenue
Concentrations of Credit Risk
Percentage of concentration	7.50%
Annualized cash rent | Customer concentration | 1350 Avenue of the Americas
Concentrations of Credit Risk
Percentage of concentration	7.20%
Annualized cash rent | Customer concentration | 555 W. 57th Street
Concentrations of Credit Risk
Percentage of concentration	6.70%
Annualized cash rent | Customer concentration | 125 Park Avenue
Concentrations of Credit Risk
Percentage of concentration	6.60%