Nature Of Business Properties Sold (Details) - USD ($) $ in Thousands		3 Months Ended					12 Months Ended
		Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, discontinued operations							$ 0	$ 0	$ 105,985
Gain on sale of real estate, continuing operations		$ 77,600	$ 24,100	$ 59,400	$ 1,500	$ 30,300	101,704	91,107	570
Disposal Group, Including Discontinued Operation, Gain (Loss) on Disposal								89,653	106,555
Country Club Towers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, continuing operations								30,277
1225 First Street [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, continuing operations	[1]							0
Munson Hill Towers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, continuing operations								51,395
Montgomery Village Center [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, continuing operations								$ 7,981
Medical Office Portfolio Transactions III & IV [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, discontinued operations	[2]								105,985
5740 Columbia Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, continuing operations									$ 570
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Dulles Station II [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, continuing operations	[3]						527
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Maryland Office Portfolio Transaction I [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, continuing operations	[4]						23,585
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Maryland Office Portfolio Transaction II [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate, continuing operations	[5]						$ 77,592

[1] Interest in land held for future development.

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

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

[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.