NOTES PAYABLE - Schedule of Long-term Debt Instruments (Details)	9 Months Ended
Sep. 30, 2019 USD ($) option	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 1,411,527,000	$ 2,196,154,000
Deferred financing costs, net	(7,250,000)	(11,616,000)
Total notes payable, net	1,404,277,000	2,184,538,000
Portfolio Loan Facility \| Secured Debt
Debt Instrument [Line Items]
Total notes payable, net	684,200,000
Portfolio Revolving Loan Facility \| Secured Debt
Debt Instrument [Line Items]
Total notes payable, net	107,500,000
Mortgages \| Portfolio Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	0	84,484,000
Mortgages \| 222 Main Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	0	97,522,000
Mortgages \| Anchor Centre Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 49,194,000	49,647,000
Effective Interest Rate	3.60%
Mortgages \| Anchor Centre Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Mortgages \| 171 17th Street Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 0	84,460,000
Mortgages \| Reston Square Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	0	29,479,000
Mortgages \| 101 South Hanley Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	0	43,090,000
Mortgages \| 3003 Washington Boulevard Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	0	90,378,000
Mortgages \| 201 17th Street Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 64,750,000	64,428,000
Effective Interest Rate	3.54%
Mortgages \| 201 17th Street Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%
Mortgages \| CrossPoint at Valley Forge Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 0	51,000,000
Mortgages \| The Almaden Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 93,000,000	93,000,000
Stated percentage	4.20%
Effective Interest Rate	4.20%
Mortgages \| Promenade I & II at Eilan Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 0	37,300,000
Mortgages \| 201 Spear Street Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 125,000,000	125,000,000
Effective Interest Rate	3.52%
Mortgages \| 201 Spear Street Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.45%
Mortgages \| Carillon Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 111,000,000	92,197,000
Effective Interest Rate	3.20%
Mortgages \| Carillon Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%
Mortgages \| 3001 Washington Boulevard Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 0	32,662,000
Mortgages \| Hardware Village Loan Facility
Debt Instrument [Line Items]
Total notes payable principal outstanding	0	49,664,000
Mortgages \| Portfolio Loan Facility
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 684,225,000	893,500,000
Effective Interest Rate	3.87%
Mortgages \| Portfolio Loan Facility \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.80%
Mortgages \| Village Center Station II Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 0	78,343,000
Mortgages \| Portfolio Revolving Loan Facility
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 141,113,000	200,000,000
Effective Interest Rate	3.60%
Mortgages \| Portfolio Revolving Loan Facility \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Mortgages \| 3001 & 3003 Washington Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 143,245,000	$ 0
Effective Interest Rate	3.55%
Mortgages \| 3001 & 3003 Washington Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.45%
Secured Debt \| Portfolio Loan Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 912,300,000
Loan, amount outstanding	684,200,000
Potential maximum borrowing capacity	1,310,000,000
Additional increase	400,000,000.0
Increments of increase	25,000,000.0
Secured Debt \| Portfolio Loan Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	228,100,000
Unused borrowing capacity, amount	$ 228,100,000
Maximum borrowing capacity, percentage	25.00%
Secured Debt \| Portfolio Loan Facility \| Non-Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity, percentage	75.00%
Secured Debt \| Portfolio Revolving Loan Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 215,000,000.0
Loan, amount outstanding	$ 107,500,000
Number of options to increase committed amount \| option	4
Potential aggregate borrowing capacity	$ 170,000,000.0
Secured Debt \| Portfolio Revolving Loan Facility \| Minimum
Debt Instrument [Line Items]
Potential borrowing capacity	15,000,000.0
Secured Debt \| Portfolio Revolving Loan Facility \| Maximum
Debt Instrument [Line Items]
Potential borrowing capacity	385,000,000.0
Secured Debt \| Portfolio Revolving Loan Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	107,500,000
Loan, amount outstanding	33,600,000
Secured Debt \| Portfolio Revolving Loan Facility \| Revolving Credit Facility \| Upon Meeting Certain Financial Coverage Ratio
Debt Instrument [Line Items]
Unused borrowing capacity, amount	$ 73,900,000
Secured Debt \| Portfolio Revolving Loan Facility \| Non-Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity, percentage	50.00%

Line of credit facility, maximum borrowing capacity, percentage.

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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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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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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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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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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, 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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