Subsequent Events (Details) (USD $)	6 Months Ended				1 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 At-the-market equity offering program	Aug. 31, 2013 Subsequent event 315 West 33rd Street Expected Completion	Jul. 31, 2013 Subsequent event At-the-market equity offering program
Subsequent Events
Common stock, shares sold and settled			95,000		165,718
Aggregate gross proceeds from shares sold			$ 8,700,000		$ 15,200,000
Net proceeds from offerings	8,478,000	201,307,000			14,900,000
Number of common partnership units exchanged (in shares)			95,000		165,718
Aggregate value of common stock remained available for issuance			36,700,000		21,500,000
Purchase consideration for mixed-use residential and commercial property				$ 386,000,000