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Real Estate Investments - Dispositions (Textual) (Details) |  | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended |
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Mar. 31, 2017 USD ($) property | Sep. 30, 2018 USD ($) ft² | Dec. 31, 2017 USD ($) a Parcel | Mar. 31, 2017 USD ($) property | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) property | Jun. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) ft² property | Dec. 31, 2018 USD ($) ft² a property Parcel | Dec. 31, 2017 USD ($) a Parcel | Dec. 31, 2016 USD ($) | Dec. 31, 2016 | Dec. 31, 2016 property | Dec. 31, 2016 a | Dec. 31, 2016 Parcel | Dec. 31, 2016 Office
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties | property |  |  |  |  |  |  |  |  | 97 |  |  |  |  |  |  |
Aggregate carrying values of properties |  |  | $ 3,832,348,000 |  |  |  |  |  | $ 3,953,319,000 | $ 3,832,348,000 |  |  |  |  |  |
Number of Properties Sold | property |  |  |  |  |  |  |  |  | 10 |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  |  |  |  |  | 1,414,457 |  |  |  |  |  |  |
Assets held for sale, net |  |  | $ 392,000 |  |  |  |  |  | $ 11,599,000 | 392,000 |  |  |  |  |  |
Net gain on disposition of real estate |  |  |  |  |  |  |  |  | 2,932,000 | 31,657,000 | $ 116,983,000 |  |  |  |  |
Impairment charges |  |  |  |  | $ 1,800,000 |  |  |  | $ 71,707,000 | $ 3,057,000 | 40,517,000 |  |  |  |  |
Land [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of land | a |  |  | 76.4 |  |  |  |  |  | 19.7 | 76.4 |  |  |  | 122.9 |  |
Number of Parcels | Parcel |  |  | 5 |  |  |  |  |  | 2 | 5 |  |  |  |  | 3 |
Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for impairment on assets held for sale |  | $ 56,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
2970 Market Street (Cira Square) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net gain on disposition of real estate |  |  |  | $ 500,000 |  |  |  |  |  |  |  |  |  |  |  |
Assets Held-for-sale [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held for sale, net |  |  |  |  |  |  |  |  | $ 11,599,000 |  | 41,718,000 |  |  |  |  |
Impairment charges |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |
Acreage of land | a |  |  | 13.1 |  |  |  |  |  |  | 13.1 |  |  |  |  |  |
Assets Held-for-sale [Member] | Land Held For Development [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held for sale, net |  |  | $ 400,000 |  |  |  |  |  |  | $ 400,000 |  |  |  |  |  |
Assets Held-for-sale [Member] | Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment Hold Period |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |  |  |
Number of Properties Sold | property |  |  |  |  |  |  |  | 8 |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  | 1,293,197 |  |  |  |  |  | 1,293,197 |  |  |  |  |  |  |  |
Assets held for sale, net |  |  |  |  |  |  |  |  |  |  | 9,785,000 |  |  |  |  |
Provision for impairment on assets held for sale |  |  |  |  |  |  |  | $ 56,900,000 |  |  |  |  |  |  |  |
Assets Held-for-sale [Member] | Metropolitan Washington, D.C. - Office [Member] | Office Properties [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held for sale, net |  | $ 366,000,000.0 |  |  |  |  |  | 366,000,000.0 |  |  |  |  |  |  |  |
Assets held for sale, fair value |  | $ 309,100,000 |  |  |  |  |  | $ 309,100,000 |  |  |  |  |  |  |  |
Residual Capitalization Rates [Member] | Assets Held-for-sale [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 |  | 0.0747 |  |  |  |  |  | 0.0747 |  |  |  |  |  |  |  |
Discount Rates [Member] | Assets Held-for-sale [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 |  | 0.0860 |  |  |  |  |  | 0.0860 |  |  |  |  |  |  |  |
Other [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties | 4 |  |  | 4 |  |  |  |  | 1 |  |  |  | 3 |  |  | 2
Impairment of properties held for use | $ 1,000,000.0 |  |  |  |  |  |  |  | $ 14,800,000 |  | 7,300,000 |  |  |  |  |
Carrying value of real estate prior to impairment |  |  | 10,200,000 |  |  |  |  |  | $ 52,800,000 | 10,200,000 | $ 25,800,000 |  |  |  |  |
Impairment Hold Period | 10 years |  |  |  |  |  |  |  | 10 years |  | 10 years |  |  |  |  |
Impairment charges |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate carrying value |  |  |  |  |  |  |  |  |  |  | $ 18,500,000 |  |  |  |  |
Other [Member] | Land [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment of properties held for use |  |  |  |  |  |  |  |  |  |  | 5,600,000 |  |  |  |  |
Carrying value of real estate prior to impairment |  |  |  |  |  |  |  |  |  |  | 18,200,000 |  |  |  |  |
Impairment charges |  |  | 0 |  |  |  | $ 300,000 |  |  |  |  |  |  |  |  |
Acreage of land | a |  |  |  |  |  |  |  |  |  |  |  |  |  | 108 |  |
Aggregate carrying value |  |  |  |  |  |  |  |  |  |  | $ 12,600,000 |  |  |  |  |
Number of Parcels | Parcel |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 |
Other [Member] | Impairment on Held for Use Properties [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate carrying values of properties |  |  | $ 9,200,000 |  |  |  |  |  | $ 38,000,000 | $ 9,200,000 |  |  |  |  |  |
Other [Member] | Residual Capitalization Rates [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 |  |  |  |  | 0.075 |  |  | 0.0875 |  |  |  |
Other [Member] | Discount Rates [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 | 0.0925 |  |  | 0.0925 |  |  |  |  | 0.095 |  |  | 0.0900 |  |  |  |
Metropolitan DC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  | 3
Impairment of properties held for use |  |  |  |  | 3,900,000 | $ 7,400,000 |  |  |  |  |  |  |  |  |  |
Carrying value of real estate prior to impairment |  |  |  |  | $ 37,400,000 | $ 51,900,000 |  |  |  |  |  |  |  |  |  |
Impairment Hold Period |  |  |  |  | 10 years | 10 years |  |  |  |  |  |  |  |  |  |
Aggregate carrying value |  |  |  |  | $ 33,500,000 | $ 44,500,000 |  |  |  |  |  |  |  |  |  |
Metropolitan DC [Member] | Residual Capitalization Rates [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 |  |  |  |  | 0.0775 | 0.070 |  |  |  |  |  |  |  |  |  |
Metropolitan DC [Member] | Discount Rates [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 |  |  |  |  | 0.0825 | 0.070 |  |  |  |  |  |  |  |  |  |

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