NOTES PAYABLE - Schedule of Long-term Debt Instruments (Details)	3 Months Ended
Mar. 31, 2020 USD ($) Investments	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 1,503,985,000	$ 1,466,376,000
Deferred financing costs, net	(6,636,000)	(6,497,000)
Total Notes Payable, net	1,497,349,000	1,459,879,000
Portfolio Loan Facility \| Secured Debt
Debt Instrument [Line Items]
Total Notes Payable, net	684,200,000
Mortgages \| Anchor Centre Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 48,892,000	49,043,000
Effective Interest Rate	3.08%
Number of extensions \| Investments	1
Extension period	6 months
Mortgages \| Anchor Centre Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Mortgages \| 201 17th Street Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 0	64,750,000
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 \| 201 Spear Street Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 125,000,000	125,000,000
Effective Interest Rate	2.83%
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	111,000,000
Effective Interest Rate	3.04%
Mortgages \| Carillon Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%
Mortgages \| Portfolio Loan Facility
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 688,225,000	684,225,000
Effective Interest Rate	3.91%
Mortgages \| Portfolio Loan Facility \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.80%
Mortgages \| Portfolio Revolving Loan Facility
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 294,623,000	196,113,000
Effective Interest Rate	3.08%
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	$ 143,245,000
Effective Interest Rate	3.03%
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
Additional increase	400,000,000.0
Increments of increase	25,000,000.0
Potential maximum borrowing capacity	$ 1,310,000,000
Number of extensions \| Investments	2
Extension period	1 year
Secured Debt \| Portfolio Loan Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 228,100,000
Loan, amount outstanding	4,000,000.0
Unused borrowing capacity, amount	$ 224,100,000
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%

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Debt Instrument, Number of Extensions

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Line of credit facility, maximum borrowing capacity, percentage.

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Line of Credit Facility,Increase in Maximum Borrowing Capacity, Increments

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

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No definition available.

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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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No definition available.

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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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

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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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