Significant Accounting Policies (Details 5)	3 Months Ended
	Mar. 31, 2014 investment
Credit concentration
Concentration of Credit Risk
Number of investments (investment)	1
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)	9.70%
Annualized rent | 1185 Avenue of the Americas | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.40%
Annualized rent | One Madison Avenue | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	5.90%
Annualized rent | Tenant 1 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.30%
Annualized rent | Tenant 2 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	6.50%
Annualized rent | Tenant 3 | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	5.80%
Revenue earned on debt and preferred equity investments | Credit concentration
Concentration of Credit Risk
Minimum revenue on debt and preferred equity investments (percent)	13.70%