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Real Estate Investments - Summary of Properties Classified as Held for Sale Included in Continuing Operations (Parenthetical) (Details) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2016 USD ($) | Jun. 30, 2016 USD ($) | Dec. 31, 2018 USD ($) property Parcel | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) property
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Provision for impairment |  | $ 1,800,000 | $ 71,707,000 | $ 3,057,000 | $ 40,517,000
Number of Properties Sold | property |  |  | 10 |  |
Held for Sale Properties Included in Continuing Operations [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Provision for impairment |  |  | $ 0 |  |
Held for Sale Properties Included in Continuing Operations [Member] | Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Number of Properties Sold | property |  |  |  |  | 3
Provision for impairment on assets held for sale | $ 3,000,000.0 |  |  |  |
Impairment Hold Period |  |  |  |  | 10 years
Held for Sale Properties Included in Continuing Operations [Member] | Other Segment - Office [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Number of Properties Sold | property |  |  |  |  | 2
Provision for impairment on assets held for sale | $ 11,500,000 |  |  |  |
Impairment Hold Period |  |  |  |  | 10 years
Held for Sale Properties Included in Continuing Operations [Member] | Residual Capitalization Rates [Member] | Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Measurement input for impairment on assets held for sale | 0.0900 |  |  |  | 0.0900
Held for Sale Properties Included in Continuing Operations [Member] | Residual Capitalization Rates [Member] | Other Segment - Office [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Measurement input for impairment on assets held for sale | 0.0975 |  |  |  | 0.0975
Held for Sale Properties Included in Continuing Operations [Member] | Discount Rates [Member] | Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Measurement input for impairment on assets held for sale | 0.1000 |  |  |  | 0.1000
Held for Sale Properties Included in Continuing Operations [Member] | Discount Rates [Member] | Other Segment - Office [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Measurement input for impairment on assets held for sale | 0.0975 |  |  |  | 0.0975
Held for Sale Properties Included in Continuing Operations [Member] | Pennsylvania Suburbs - Land [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Number of Parcels | Parcel |  |  | 1 |  |
Held for Sale Properties Included in Continuing Operations [Member] | Other - Land [Member] |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |
Number of Parcels | Parcel |  |  | 2 |  |

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