Real Estate (Significant Acquisitions and Disposition) (Details) (USD $)	3 Months Ended	12 Months Ended			0 Months Ended				24 Months Ended	0 Months Ended
	Sep. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 05, 2013	Jul. 22, 2013	Sep. 10, 2013	Jan. 02, 2014	Dec. 31, 2014	Apr. 03, 2013	Oct. 02, 2013 acre	Dec. 19, 2013
Business Acquisition [Line Items]
Mortgage loans assumed with acquisition		$ 68,282,000	$ 0	$ 67,615,000
Contributions from noncontrolling interests		65,350,000		175,000
Gain on sale of real estate		4,401,000	4,994,000	11,860,000
Discontinued operations - gain on sale of real estate	23,900,000	0	23,861,000	0
Shirlington
Business Acquisition [Line Items]
Proceeds from sale					6,500,000
Gain on sale of real estate					5,000,000
Fifth Avenue
Business Acquisition [Line Items]
Proceeds from sale						15,300,000
Discontinued operations - gain on sale of real estate						10,700,000
Forest Hills
Business Acquisition [Line Items]
Proceeds from sale							20,400,000
Discontinued operations - gain on sale of real estate							13,200,000
Land Parcel Adjacent to Assembly Row
Business Acquisition [Line Items]
Area of land											11.8
Land acquisition purchase price											18,000,000
Land Contiguous with Montrose Crossing
Business Acquisition [Line Items]
Land acquisition purchase price												10,500,000
Land acquistion, share of purchase price												9,400,000
Montrose Crossing
Business Acquisition [Line Items]
Ownership percentage												89.90%
The Grove at Shrewsbury
Business Acquisition [Line Items]
Leasable area (square feet)								187,000
Mandatorily redeemable noncontrolling interested classified as a liability		9,000,000							9,000,000
Brook 35
Business Acquisition [Line Items]
Leasable area (square feet)								99,000
Grove at Shrewsbury and Brook 35
Business Acquisition [Line Items]
Gross value								161,000,000
Effective economic interest percentage								84.00%
Mortgage loans assumed with acquisition								68,000,000
Number of downREIT operating partnership units								632,000
Cash payments to acquire businesses								13,000,000
Net assets acquired allocated to below market leases								2,300,000
Contributions from noncontrolling interests								71,100,000
Acquisition costs		1,000,000	1,000,000						2,000,000
Darien
Business Acquisition [Line Items]
Leasable area (square feet)										95,000
Net assets acquired allocated to below market leases										1,800,000
Acquisition costs			200,000
Business acquisition purchase price										47,300,000
Above Market Leases | Grove at Shrewsbury and Brook 35
Business Acquisition [Line Items]
Net assets acquired allocated to above market leases								1,700,000
Above Market Leases | Darien
Business Acquisition [Line Items]
Net assets acquired allocated to above market leases										$ 100,000