Real Estate Investments (Schedule of Annualized Rental Income by Major Tenant) (Details)	12 Months Ended
	Dec. 31, 2012 229 West 36th Street [Member] American Language Communication Center, Inc. [Member]	Dec. 31, 2011 One Jackson Square [Member] TD Bank [Member]	Dec. 31, 2011 Duane Reed [Member]	Dec. 31, 2011 Bleecker Street [Member] Burberry Limited [Member]	Dec. 31, 2010 Bleecker Street [Member] Burberry Limited [Member]	Dec. 31, 2010 Bleecker Street [Member] Michael Kors Stores, LLC [Member]	Dec. 31, 2010 Interior Design Building [Member] Rosselli 61st St, LLC [Member]
Revenue, Major Customer [Line Items]
Major tenant rental income, as a percentage of total annualized rental income	13.50%	12.20%	10.00%	10.70%	17.30%	10.40%	11.30%