Real Estate Investments - Schedule of Properties Sold (Details) $ in Thousands		3 Months Ended					12 Months Ended
		Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($) ft² apartment	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) ft² unit	Dec. 31, 2015 USD ($) ft² apartment	Dec. 31, 2014 USD ($) ft²
Real Estate [Line Items]
Units				506			4,480	506
Area of Real Estate Property | ft²				197,000			1,183,000	197,000	430,000
Disposal Group, Including Discontinued Operation, Consideration				$ 137,100			$ 252,100	$ 137,100	$ 195,161
Gain on sale of real estate, continuing operations		$ 77,600	$ 24,100	$ 59,400	$ 1,500	$ 30,300	101,704	91,107	570
Gain on sale of real estate, discontinued operations							0	0	105,985
Disposal Group, Including Discontinued Operation, Gain (Loss) on Disposal								89,653	106,555
Dulles Station II [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Real Estate [Line Items]
Gain on sale of real estate, continuing operations	[1]						527
Dulles Station II [Member] | Office [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Real Estate [Line Items]
Disposal Group, Including Discontinued Operation, Consideration	[2]						12,100
Maryland Office Portfolio Transaction I [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Real Estate [Line Items]
Gain on sale of real estate, continuing operations	[3]						$ 23,585
Maryland Office Portfolio Transaction I [Member] | Office [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Real Estate [Line Items]
Area of Real Estate Property | ft²							692,000
Disposal Group, Including Discontinued Operation, Consideration	[4]						$ 111,500
Maryland Office Portfolio Transaction II [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Real Estate [Line Items]
Gain on sale of real estate, continuing operations	[5]						$ 77,592
Maryland Office Portfolio Transaction II [Member] | Office [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Real Estate [Line Items]
Area of Real Estate Property | ft²							491,000
Disposal Group, Including Discontinued Operation, Consideration							$ 128,500
Country Club Towers [Member]
Real Estate [Line Items]
Gain on sale of real estate, continuing operations								$ 30,277
Country Club Towers [Member] | Multifamily [Member]
Real Estate [Line Items]
Units | apartment				227				227
Disposal Group, Including Discontinued Operation, Consideration				$ 37,800				$ 37,800
1225 First Street [Member]
Real Estate [Line Items]
Gain on sale of real estate, continuing operations	[6]							0
1225 First Street [Member] | Multifamily [Member]
Real Estate [Line Items]
Disposal Group, Including Discontinued Operation, Consideration	[7]			$ 14,500				14,500
Munson Hill Towers [Member]
Real Estate [Line Items]
Gain on sale of real estate, continuing operations								$ 51,395
Munson Hill Towers [Member] | Multifamily [Member]
Real Estate [Line Items]
Units | apartment				279				279
Disposal Group, Including Discontinued Operation, Consideration				$ 57,050				$ 57,050
Montgomery Village Center [Member]
Real Estate [Line Items]
Gain on sale of real estate, continuing operations								$ 7,981
Montgomery Village Center [Member] | Retail [Member]
Real Estate [Line Items]
Area of Real Estate Property | ft²				197,000				197,000
Disposal Group, Including Discontinued Operation, Consideration				$ 27,750				$ 27,750
Medical Office Portfolio Transactions III & IV [Member]
Real Estate [Line Items]
Gain on sale of real estate, discontinued operations	[8]								$ 105,985
Medical Office Portfolio Transactions III & IV [Member] | Medical Office Building [Member]
Real Estate [Line Items]
Area of Real Estate Property | ft²									427,000
Disposal Group, Including Discontinued Operation, Consideration	[9]								$ 193,561
5740 Columbia Road [Member]
Real Estate [Line Items]
Gain on sale of real estate, continuing operations									$ 570
5740 Columbia Road [Member] | Retail [Member]
Real Estate [Line Items]
Area of Real Estate Property | ft²									3,000
Disposal Group, Including Discontinued Operation, Consideration									$ 1,600

[1] Land held for future development and an interest in a parking garage.

[2] Land held for future development and an interest in a parking garage.

[3] Maryland Office Portfolio Transaction I consists of 6110 Executive Boulevard, 600 Jefferson Plaza, Wayne Plaza and West Gude Drive.

[4] Maryland Office Portfolio Transaction I consists of 6110 Executive Boulevard, 600 Jefferson Plaza, Wayne Plaza and West Gude Drive.

[5] Maryland Office Portfolio Transaction II consists of 51 Monroe Street and One Central Plaza.

[6] Interest in land held for future development.

[7] Interest in land held for future development.

[8] Medical Office Portfolio Transactions III & IV consist of Woodburn Medical Park I and II and Prosperity Medical Center I, II and III.

[9] These properties are classified as discontinued operations. All other sold properties are classified as continuing operations.