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Equity method investment summarized financial information (gain) loss on extinguishment of debt.

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Lessee under ground lease term.

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Amount, after allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business.

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Area of land held.

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Amount of the cumulative effect on retained earnings net of related income tax effect.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Contractual interest rate for funds borrowed, under the debt agreement.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

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The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

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Net rentable area for properties owned.

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The number of units in a real estate property owned as of the balance sheet date.

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The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

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Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

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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.

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Investment in Unconsolidated Real Estate Ventures (Textual) (Details)					3 Months Ended		9 Months Ended
	Apr. 12, 2019 USD ($)	Apr. 01, 2019 USD ($)	Mar. 29, 2019 USD ($) property	Jan. 01, 2019 USD ($)	Sep. 30, 2019 USD ($) a ft² apartment investment	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) a ft² parcel apartment investment	Sep. 30, 2018 USD ($)	Aug. 29, 2019 USD ($) ft²	Apr. 11, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Accounts receivable					$ 13,872,000		$ 13,872,000				$ 16,394,000
Cumulative effect of accounting change				$ (5,336,000)
Minimum
Schedule of Equity Method Investments [Line Items]
Ownership percentage					15.00%		15.00%
Maximum
Schedule of Equity Method Investments [Line Items]
Ownership percentage					70.00%		70.00%
Real Estate Venture
Schedule of Equity Method Investments [Line Items]
Accounts receivable					$ 800,000		$ 800,000				$ 800,000
Real Estate Venture \| Management Fees
Schedule of Equity Method Investments [Line Items]
Service and other revenue					1,000,000.0	$ 1,400,000	3,200,000	$ 4,000,000.0
Real Estate Venture \| Leasing Commission Income
Schedule of Equity Method Investments [Line Items]
Service and other revenue					300,000	1,500,000	800,000	4,400,000
Unconsolidated Real Estate Ventures
Schedule of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method					127,800,000		127,800,000
Gain on early extinguishment of debt					4,371,000	$ (334,000)	4,371,000	$ (1,052,000)
Real estate ventures aggregate indebtedness					$ 592,400,000		$ 592,400,000
Unconsolidated Real Estate Ventures \| Minimum
Schedule of Equity Method Investments [Line Items]
Ownership percentage					15.00%	25.00%	15.00%	25.00%
Unconsolidated Real Estate Ventures \| Maximum
Schedule of Equity Method Investments [Line Items]
Ownership percentage					70.00%	70.00%	70.00%	70.00%
Unconsolidated Real Estate Ventures \| Six Real Estate Ventures \| Office properties
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures \| investment					6		6
Rentable area (in square feet) \| ft²					5,600,000		5,600,000
Unconsolidated Real Estate Ventures \| Two Real Estate Ventures \| Land Held For Development
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures \| investment					2		2
Area of land (in acres) \| a					1.4		1.4
Unconsolidated Real Estate Ventures \| One Real Estate Venture \| Land Under Active Development
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures \| investment					1		1
Area of land (in acres) \| a					1.3		1.3
Unconsolidated Real Estate Ventures \| One Other Real Estate Venture \| Residential Tower
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures \| investment					1		1
Number of Property units \| apartment					321		321
Falls Church, Virginia
Schedule of Equity Method Investments [Line Items]
Rentable area (in square feet) \| ft²									180,659
Mortgage loan on real estate									$ 26,000,000.0
Gain on early extinguishment of debt					$ 2,200,000
MAP Venture
Schedule of Equity Method Investments [Line Items]
Ownership percentage					50.00%	50.00%	50.00%	50.00%
Gain on early extinguishment of debt					$ 0	$ (334,000)	$ 0	$ (334,000)
Number of lessee land parcels \| parcel							58
Lessee under ground lease term							99 years
Cumulative effect of accounting change				$ 9,200,000			$ 4,600,000
Herndon Innovation Center Metro Portfolio Venture, LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage			15.00%
Herndon Innovation Center Metro Portfolio Venture, LLC \| Secured Debt
Schedule of Equity Method Investments [Line Items]
Indebtedness to third parties, refinanced amount			$ 134,100,000
Number of properties used to secure debt \| property			4
Debt instrument, initial advance		$ 111,000,000.0	$ 113,100,000
Herndon Innovation Center Metro Portfolio Venture, LLC \| Secured Debt \| Distributions from Loan 1
Schedule of Equity Method Investments [Line Items]
Debt instrument, initial advance	$ 16,700,000
Spread on variable rate							1.95%
Herndon Innovation Center Metro Portfolio Venture, LLC \| Secured Debt \| Distributions from Loan 2
Schedule of Equity Method Investments [Line Items]
Indebtedness to third parties, refinanced amount										$ 115,300,000
Debt instrument, initial advance	$ 13,800,000
Spread on variable rate							1.80%
Herndon Innovation Center Metro Portfolio Venture, LLC \| Minimum \| Secured Debt \| Distributions from Loan 1
Schedule of Equity Method Investments [Line Items]
Effective interest rate					5.45%		5.45%
Herndon Innovation Center Metro Portfolio Venture, LLC \| Maximum \| Secured Debt \| Distributions from Loan 1
Schedule of Equity Method Investments [Line Items]
Effective interest rate					6.45%		6.45%
Herndon Innovation Center Metro Portfolio Venture, LLC \| Maximum \| Secured Debt \| Distributions from Loan 2
Schedule of Equity Method Investments [Line Items]
Debt instrument, interest rate, stated percentage					6.30%		6.30%
PJP VII
Schedule of Equity Method Investments [Line Items]
Construction loan, principal amount					$ 3,700,000		$ 3,700,000
Guarantees, maximum exposure amount					300,000		300,000
4040 Wilson Venture
Schedule of Equity Method Investments [Line Items]
Indebtedness to third parties, refinanced amount					150,000,000.0		150,000,000.0
Guarantees, maximum exposure amount					$ 41,300,000		$ 41,300,000