Significant Accounting Policies (Details 6)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Workforce concentration
Concentration of Credit Risk
Number of collective bargaining agreements (agreement)	6
Annualized rent | Customer concentration
Concentration of Credit Risk
Number of tenants (tenants)	3
Maximum percentage of annualized rent for any one tenant not individually disclosed (percent)	2.00%
Annualized rent | 1515 Broadway | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	9.90%	10.60%	10.20%
Annualized rent | 388 and 390 Greenwich Street | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	9.80%
Annualized rent | 919 Third Avenue | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.50%	7.80%	8.00%
Annualized rent | 1185 Avenue of the Americas | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.80%	7.70%	7.10%
Annualized rent | One Madison Avenue | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)		6.40%	6.20%
Annualized rent | Tenant 1 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	10.90%
Annualized rent | Tenant 2 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.20%
Annualized rent | Tenant 3 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	5.60%
Collective bargaining arrangements | Workforce concentration
Concentration of Credit Risk
Percentage of concentration (percent)	73.40%
Collective bargaining arrangements which expires in 2015 | Workforce concentration
Concentration of Credit Risk
Percentage of concentration (percent)	77.60%