Number of parcels of land.

No definition available.

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- Definition

The number of real estate properties sold during the period.

+ References

No definition available.

+ Details

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- Definition

Expected hold period for assets held for use, used as an input to measure fair value

+ References

No definition available.

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- Definition

Value of input used to measure alternative investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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- Definition

The aggregate amount of write-downs for impairments recognized during the period for long-lived assets held for abandonment, exchange or sale.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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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
	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