Real Estate Activity During the Six Months Ended June 30, 2015 (Narrative) (Details) $ in Thousands			3 Months Ended			6 Months Ended
	Mar. 17, 2015 USD ($) ft² apartments	Feb. 19, 2015 USD ($) a	Jun. 30, 2015 USD ($)	Mar. 16, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	May. 01, 2015 ft²
Real Estate Properties [Line Items]
Operating income			$ 204,926		$ 201,273	$ 411,135	$ 378,170
Proceeds from sales of real estate						194,821	0
Gains on sales of real estate			$ 0		0	$ 95,084	0
Washingtonian North land parcel [Member]
Real Estate Properties [Line Items]
Sale Price Of Sold Property		$ 8,700
Proceeds from sales of real estate		8,300
Gains on sales of real estate		$ 3,700
Land sold (in acres) | a		8.5
Area of Land (in acres) | a		27
Residences on The Avenue [Member]
Real Estate Properties [Line Items]
Operating income				$ 1,100	$ 900		$ 1,700
Rental revenue supoprt payments to buyer maximum	$ 6,000
Sale Price Of Sold Property	196,000
Proceeds from sales of real estate	192,500
Gains on sales of real estate	$ 91,400
Number of apartment units (apartment units) | apartments	335
Reservoir Place North [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²								73,000
Retail Site [Member] | Residences on The Avenue [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²	50,000