NOTES PAYABLE - Schedule of Long-term Debt Instruments (Details)	6 Months Ended
Jun. 30, 2021 USD ($) extensionOption	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 1,396,745,000	$ 1,396,745,000
Deferred financing costs, net	(6,437,000)	(8,380,000)
Total Notes Payable, net	1,390,308,000	1,388,365,000
Carillon Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	88,800,000
Carillon Mortgage Loan \| Revolving Credit Facility
Debt Instrument [Line Items]
Unused borrowing capacity, amount	22,200,000
Modified Portfolio Loan Facility \| Secured Debt
Debt Instrument [Line Items]
Total Notes Payable, net	472,900,000
Modified Portfolio Revolving Loan Facility \| Secured Debt
Debt Instrument [Line Items]
Total Notes Payable, net	162,500,000
Accenture Tower Revolving Loan \| Secured Debt
Debt Instrument [Line Items]
Total Notes Payable, net	281,300,000
Mortgages \| The Almaden Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 123,000,000	123,000,000
Stated percentage	3.65%
Effective Interest Rate	3.65%
Extension period	12 months
Number of extensions \| extensionOption	2
Mortgages \| The Almaden Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	3.50%
Mortgages \| 201 Spear Street Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 125,000,000	125,000,000
Effective Interest Rate	1.55%
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	$ 88,800,000	88,800,000
Effective Interest Rate	1.50%
Mortgages \| Carillon Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%
Mortgages \| 3001 & 3003 Washington Mortgage Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 143,245,000	143,245,000
Effective Interest Rate	1.55%
Mortgages \| 3001 & 3003 Washington Mortgage Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.45%
Secured Debt \| Modified Portfolio Loan Facility
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 472,950,000	472,950,000
Effective Interest Rate	1.90%
Maximum borrowing capacity	$ 630,600,000
Additional number of extensions \| extensionOption	1
Extension period	12 months
Secured Debt \| Modified Portfolio Loan Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 157,700,000
Secured Debt \| Modified Portfolio Loan Facility \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.80%
Secured Debt \| Modified Portfolio Revolving Loan Facility
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 162,500,000	162,500,000
Effective Interest Rate	1.60%
Extension period	12 months
Number of extensions \| extensionOption	2
Secured Debt \| Modified Portfolio Revolving Loan Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 162,500,000
Secured Debt \| Modified Portfolio Revolving Loan Facility \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Secured Debt \| Accenture Tower Revolving Loan
Debt Instrument [Line Items]
Total notes payable principal outstanding	$ 281,250,000	$ 281,250,000
Effective Interest Rate	2.35%
Secured Debt \| Accenture Tower Revolving Loan \| Revolving Credit Facility
Debt Instrument [Line Items]
Unused borrowing capacity, amount	$ 93,700,000
Extension period	12 months
Number of extensions \| extensionOption	2
Secured Debt \| Accenture Tower Revolving Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%

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