Significant Accounting Policies (Details 5)	3 Months Ended
	Mar. 31, 2013 item
Concentrations of Credit Risk
Number of tenants	3
Annualized rent | Customer concentration
Concentrations of Credit Risk
Maximum percentage of annualized rent from other than Three tenants	1.90%
Annualized rent | 1515 Broadway | Customer concentration
Concentrations of Credit Risk
Maximum percentage of annualized rent from other than Three tenants	9.00%
Annualized rent | 1185 Avenue of the Americas | Customer concentration
Concentrations of Credit Risk
Maximum percentage of annualized rent from other than Three tenants	7.00%
Annualized rent | 420 Lexington Avenue | Customer concentration
Concentrations of Credit Risk
Maximum percentage of annualized rent from other than Three tenants	6.00%
Annualized rent | One Madison Avenue | Customer concentration
Concentrations of Credit Risk
Maximum percentage of annualized rent from other than Three tenants	6.00%
Annualized rent | Tenant one | Customer concentration
Concentrations of Credit Risk
Percentage of concentration	6.00%
Annualized rent | Tenant two | Customer concentration
Concentrations of Credit Risk
Percentage of concentration	6.50%
Annualized rent | Tenant three | Customer concentration
Concentrations of Credit Risk
Percentage of concentration	7.00%
Revenue earned on debt and preferred equity investments | Two debt and preferred equity investments | Credit concentration
Concentrations of Credit Risk
Number of investments	2
Revenue earned on debt and preferred equity investments | Two debt and preferred equity investments | Credit concentration | Minimum
Concentrations of Credit Risk
Percentage of concentration	10.00%