Significant Accounting Policies (Details 6)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Credit concentration
Concentration of Credit Risk
Number of investments (investment)	3
Workforce concentration
Concentration of Credit Risk
Number of collective bargaining agreements (agreement)	3
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)	2.20%
Annualized rent | 1515 Broadway | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	10.60%	10.20%	10.30%
Annualized rent | 919 Third Avenue | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.80%	8.00%	8.30%
Annualized rent | 1185 Avenue of the Americas | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.70%	7.10%	7.20%
Annualized rent | One Madison Avenue | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	6.40%	6.20%	6.50%
Annualized rent | Tenant 1 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.50%
Annualized rent | Tenant 2 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	6.60%
Annualized rent | Tenant 3 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	6.10%
Revenue earned on debt and preferred equity investments | Credit concentration
Concentration of Credit Risk
Minimum revenue on debt and preferred equity investments (percent)	10.00%
Collective bargaining arrangements | Workforce concentration
Concentration of Credit Risk
Percentage of concentration (percent)	73.90%
Collective bargaining arrangements which expires in 2015 | Workforce concentration
Concentration of Credit Risk
Percentage of concentration (percent)	77.30%