Significant Accounting Policies (Details 6)	6 Months Ended	3 Months Ended					6 Months Ended
	Jun. 30, 2013 Customer concentration item	Jun. 30, 2013 Credit concentration investment	Jun. 30, 2013 Annualized rent 1515 Broadway Customer concentration	Jun. 30, 2013 Annualized rent 1185 Avenue of the Americas Customer concentration	Jun. 30, 2013 Annualized rent One Madison Avenue Customer concentration	Jun. 30, 2013 Annualized rent Three tenant Customer concentration	Jun. 30, 2013 Annualized rent Tenant 1 Customer concentration	Jun. 30, 2013 Annualized rent Tenant 2 Customer concentration	Jun. 30, 2013 Annualized rent Tenant 3 Customer concentration
Concentration of Credit Risk
Number of tenants	3
Maximum percentage of annualized rent for any one tenant not individually disclosed						1.90%
Percentage of concentration			9.00%	6.00%	6.00%		6.00%	6.40%	7.10%
Number of investments		2
Minimum revenue on debt and preferred equity investments (as a percent)		10.00%