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Debt Instrument, Amortization Period

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Debt Instrument Covenant, Required Net Assets

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Debt Instrument Percentage of Net Proceeds of Additional Equity Issuance

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Interest Coverage Ratio Required Minimum

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Letter of Credit Facilities, Maximum Borrowing Capacity

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Line of Credit Facility Potential Term Extension Period

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Long-term Debt, Repayments of Principal and Balloon Payments

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Required Fixed Charge Coverage Ratio

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

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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

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

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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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Amount of the required periodic payments including both interest and principal payments.

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Amount of payment greater than the preceding installment payments to be paid at final maturity date of debt.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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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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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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Including the current and noncurrent portions, aggregate carrying value as of the balance sheet date of loans payable (with maturities initially due after one year or beyond the operating cycle if longer).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

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The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time.

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The carrying amount, as of the date of the latest financial statement presented, of real estate owned but transferred to serve as collateral for the payment of the related debt obligation, primarily a secured borrowing or repurchase agreement, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Amount of collateralized debt obligations classified as other.

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Mortgage Notes Payable, Revolving Credit Facility, Interest Expense and Amortization of Deferred Debt Costs - Additional Information (Detail) - USD ($)						1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 20, 2017	Aug. 14, 2017	Jan. 18, 2017	Apr. 01, 2015	Mar. 03, 2015	Sep. 30, 2017	Dec. 31, 2021	Dec. 31, 2017	Sep. 01, 2017	Dec. 31, 2016	Nov. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Outstanding debt								$ 965,500,000
Fixed rate mortgages								897,888,000		$ 783,400,000
Principal amount								965,528,000		907,800,000
Debt outstanding with fixed-rate										844,300,000
Debt outstanding with variable-rate										63,500,000
Remaining loan balance								284,627,000
Capitalization of debt issuance cost								2,600,000		100,000		$ 300,000
3.69% Due March 2030 \| Shops at Fairfax and Boulevard
Debt Instrument [Line Items]
Loan term					15 years
Loan amount					$ 30,000,000
Loan fixed interest rate					3.69%
Loan required periodic principal and interest payment					$ 153,300
Amortization schedule					25 years
Balloon payment to be paid					$ 15,500,000
3.99% Due April 2030 \| Northrock
Debt Instrument [Line Items]
Loan term				15 years
Loan amount				$ 16,000,000
Loan fixed interest rate				3.99%
Mortgage Notes Payable
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure								$ 51,000,000		51,000,000
Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Extension in line of credit facility period								1 year
Available borrowing capacity								$ 213,800,000
Outstanding line of credit								61,000,000
Letter of credit available borrowing capacity								$ 185,000
Interest rate spread on LIBOR								1.45%
Revolving Credit Facility \| Line of Credit \| Minimum
Debt Instrument [Line Items]
Interest rate spread on LIBOR								1.45%
Revolving Credit Facility \| Line of Credit \| Maximum
Debt Instrument [Line Items]
Interest rate spread on LIBOR								2.00%
Fixed rate mortgage notes payable
Debt Instrument [Line Items]
Fixed rate mortgages								$ 890,393,000		844,292,000
Fixed rate mortgage notes payable \| Shops at Fairfax and Boulevard
Debt Instrument [Line Items]
Remaining loan balance					$ 15,200,000
Fixed rate mortgage notes payable \| Northrock
Debt Instrument [Line Items]
Remaining loan balance				$ 14,500,000
Fixed rate mortgage notes payable \| Washington Square
Debt Instrument [Line Items]
Remaining loan balance	$ 28,100,000
Fixed rate mortgage notes payable \| 3.69% Due March 2030
Debt Instrument [Line Items]
Fixed rate mortgages								27,884,000		28,679,000
Fixed rate mortgage notes payable \| 3.99% Due April 2030
Debt Instrument [Line Items]
Fixed rate mortgages								14,950,000		15,357,000
Fixed rate mortgage notes payable \| 3.99% Due April 2030 \| Northrock
Debt Instrument [Line Items]
Loan required periodic principal and interest payment				$ 84,400
Amortization schedule				25 years
Balloon payment to be paid				$ 8,400,000
Fixed rate mortgage notes payable \| 3.51% due June 2028
Debt Instrument [Line Items]
Fixed rate mortgages								39,968,000		41,753,000
Fixed rate mortgage notes payable \| 3.51% due June 2028 \| Beacon Center
Debt Instrument [Line Items]
Loan amount											$ 11,250,000.00
Loan required periodic principal and interest payment								268,500
Fixed rate mortgage notes payable \| 3.39% due January 2032
Debt Instrument [Line Items]
Fixed rate mortgages								39,140,000		0
Fixed rate mortgage notes payable \| 4.88% due September 2032
Debt Instrument [Line Items]
Fixed rate mortgages								71,211,000		70,144,000
Fixed rate mortgage notes payable \| 3.75% due December 2032
Debt Instrument [Line Items]
Fixed rate mortgages								60,000,000		0
Variable rate loans payable
Debt Instrument [Line Items]
Variable rate debt								$ 75,135,000		63,482,000
Interest rate spread on LIBOR								2.86%
Variable rate loans payable \| Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity								$ 275,000,000
Metro Pike Center Bank Loan \| Fixed rate mortgage notes payable
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure								7,800,000
Remaining loan balance								14,100,000
Kentlands Square II Bank Loan \| Fixed rate mortgage notes payable
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure								9,200,000
Remaining loan balance								36,500,000
Burtonsville Fixed Rate Mortgage \| 3.39% due January 2032
Debt Instrument [Line Items]
Loan term			15 years
Loan amount			$ 40,000,000
Loan fixed interest rate			3.39%
Loan required periodic principal and interest payment			$ 197,900
Amortization schedule			25 years
Balloon payment to be paid			$ 20,300,000
Glebe Road Construction Loan \| 4.67% Construction To Permanent Loan Due 2035
Debt Instrument [Line Items]
Loan amount		$ 157,000,000.0
Loan fixed interest rate		4.67%
Amortization schedule		25 years
Park Van Ness Loan \| Fixed rate mortgage notes payable
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure								53,700,000
Remaining loan balance								71,200,000
Park Van Ness Loan \| 4.88% due September 2032
Debt Instrument [Line Items]
Fixed rate mortgages									$ 71,600,000
Loan fixed interest rate									4.88%
Loan required periodic principal and interest payment						$ 413,460
Amortization schedule						25 years
Balloon payment to be paid						$ 39,600,000
Washington Square 2017 Mortgage \| 3.75% due December 2032
Debt Instrument [Line Items]
Loan term	15 years
Loan amount	$ 60,000,000
Loan fixed interest rate	3.75%
Loan required periodic principal and interest payment	$ 308,500
Amortization schedule	25 years
Balloon payment to be paid	$ 31,100,000
Debt Covenant
Debt Instrument [Line Items]
Carrying value of properties collateralized								$ 1,000,000,000		$ 957,200,000
Debt covenant, percentage of net proceeds of additional equity issuance								80.00%
Credit agreement leverage ratio								60.00%
Interest coverage ratio required minimum								2.0
Required fixed charge coverage ratio								1.3
Debt Covenant \| Minimum
Debt Instrument [Line Items]
Debt covenant required net assets								$ 542,100,000
Scenario, Forecast \| Glebe Road Construction Loan \| 4.67% Construction To Permanent Loan Due 2035
Debt Instrument [Line Items]
Loan required periodic principal and interest payment							$ 887,900