NOTES PAYABLE (Schedule of Long-term Debt Instruments) (Details) - USD ($) | 6 Months Ended | | |
Jun. 30, 2017 | Feb. 23, 2017 | Dec. 31, 2016 |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 1,885,591,000 | | $ 1,793,405,000 |
Deferred financing costs, net | (7,725,000) | | (9,937,000) |
Total notes payable, net | 1,877,866,000 | | 1,783,468,000 |
Portfolio Loan | Secured Debt | | | |
Debt Instrument [Line Items] | | | |
Total notes payable, net | 127,500,000 | | |
500 West Madison Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable, net | 235,000,000 | | |
500 West Madison Mortgage Loan | First Non-Revolver Tranche | | | |
Debt Instrument [Line Items] | | | |
Current borrowing capacity | 20,000,000 | | |
Hardware Village Loan Facility | | | |
Debt Instrument [Line Items] | | | |
Maximum borrowing capacity | | $ 74,000,000 | |
Mortgages | Town Center Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 75,000,000 | | 75,000,000 |
Effective Interest Rate | 2.87% | | |
Maturity Date | Mar. 27, 2018 | | |
Mortgages | Town Center Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.85% | | |
Mortgages | Portfolio Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 163,460,000 | | 127,500,000 |
Effective Interest Rate | 2.95% | | |
Maturity Date | Jun. 1, 2019 | | |
Mortgages | Portfolio Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.90% | | |
Mortgages | RBC Plaza Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 75,647,000 | | 75,930,000 |
Effective Interest Rate | 2.85% | | |
Maturity Date | Feb. 1, 2018 | | |
Mortgages | RBC Plaza Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.80% | | |
Mortgages | National Office Portfolio Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 170,602,000 | | 170,602,000 |
Effective Interest Rate | 2.76% | | |
Maturity Date | Jul. 1, 2018 | | |
Mortgages | National Office Portfolio Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.50% | | |
Mortgages | 500 West Madison Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 235,000,000 | | 215,000,000 |
Effective Interest Rate | 3.12% | | |
Maturity Date | Dec. 16, 2018 | | |
Mortgages | 500 West Madison Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.65% | | |
Mortgages | 222 Main Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 100,416,000 | | 101,343,000 |
Stated percentage | 3.97% | | |
Effective Interest Rate | 3.97% | | |
Maturity Date | Mar. 1, 2021 | | |
Mortgages | Anchor Centre Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 50,000,000 | | 50,000,000 |
Effective Interest Rate | 3.18% | | |
Maturity Date | Jun. 1, 2018 | | |
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 | $ 83,778,000 | | 83,778,000 |
Effective Interest Rate | 2.79% | | |
Maturity Date | Sep. 1, 2018 | | |
Mortgages | 171 17th Street Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.45% | | |
Mortgages | Reston Square Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 29,800,000 | | 23,840,000 |
Effective Interest Rate | 3.59% | | |
Maturity Date | Feb. 1, 2018 | | |
Mortgages | Reston Square Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.50% | | |
Mortgages | Ten Almaden Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 66,555,000 | | 65,853,000 |
Effective Interest Rate | 3.42% | | |
Maturity Date | Jan. 1, 2018 | | |
Mortgages | Ten Almaden Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.65% | | |
Mortgages | Towers at Emeryville Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 153,524,000 | | 145,379,000 |
Effective Interest Rate | 3.95% | | |
Maturity Date | Jan. 15, 2018 | | |
Loan, amount outstanding | $ 153,500,000 | | |
Unused borrowing capacity, amount | $ 21,500,000 | | |
Mortgages | Towers at Emeryville Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.75% | | |
Mortgages | 101 South Hanley Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 40,557,000 | | 37,502,000 |
Effective Interest Rate | 3.73% | | |
Maturity Date | Jan. 1, 2020 | | |
Mortgages | 101 South Hanley Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.55% | | |
Mortgages | 3003 Washington Boulevard Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 90,378,000 | | 90,378,000 |
Effective Interest Rate | 3.54% | | |
Maturity Date | Feb. 1, 2020 | | |
Mortgages | 3003 Washington Boulevard Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.55% | | |
Mortgages | Rocklin Corporate Center Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 21,689,000 | | 20,868,000 |
Effective Interest Rate | 2.55% | | |
Maturity Date | Jun. 5, 2018 | | |
Mortgages | Rocklin Corporate Center 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 | $ 63,950,000 | | 58,063,000 |
Effective Interest Rate | 2.45% | | |
Maturity Date | Aug. 1, 2018 | | |
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 | $ 51,000,000 | | 51,000,000 |
Effective Interest Rate | 3.33% | | |
Maturity Date | Sep. 1, 2022 | | |
Mortgages | CrossPoint at Valley Forge Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.50% | | |
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% | | |
Maturity Date | Oct. 1, 2016 | | |
Mortgages | Promenade I & II at Eilan Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 37,300,000 | | 37,300,000 |
Effective Interest Rate | 3.57% | | |
Maturity Date | Oct. 1, 2022 | | |
Mortgages | Promenade I & II at Eilan Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.75% | | |
Mortgages | 515 Congress Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 67,500,000 | | 67,500,000 |
Effective Interest Rate | 2.76% | | |
Maturity Date | Nov. 1, 2020 | | |
Mortgages | 515 Congress Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.70% | | |
Mortgages | 201 Spear Street Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 100,000,000 | | 100,000,000 |
Effective Interest Rate | 2.72% | | |
Maturity Date | Jan. 1, 2019 | | |
Mortgages | 201 Spear Street Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.66% | | |
Mortgages | Carillon Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 89,306,000 | | 76,440,000 |
Effective Interest Rate | 3.23% | | |
Maturity Date | Feb. 1, 2020 | | |
Mortgages | Carillon Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.65% | | |
Mortgages | 3001 Washington Boulevard Mortgage Loan | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 27,129,000 | | 27,129,000 |
Effective Interest Rate | 2.66% | | |
Maturity Date | Feb. 1, 2019 | | |
Mortgages | 3001 Washington Boulevard Mortgage Loan | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 1.60% | | |
Mortgages | Hardware Village Loan Facility | | | |
Debt Instrument [Line Items] | | | |
Total notes payable principal outstanding | $ 0 | | $ 0 |
Maturity Date | Feb. 23, 2020 | | |
Mortgages | Hardware Village Loan Facility | One-month LIBOR | | | |
Debt Instrument [Line Items] | | | |
Basis spread on variable rate | 3.25% | | |
Secured Debt | Portfolio Loan | | | |
Debt Instrument [Line Items] | | | |
Maximum borrowing capacity | $ 255,000,000 | | |
Loan, amount outstanding | 127,500,000 | | |
Potential maximum borrowing capacity | 350,000,000 | | |
Secured Debt | Portfolio Loan | Revolving Credit Facility | | | |
Debt Instrument [Line Items] | | | |
Maximum borrowing capacity | 127,500,000 | | |
Loan, amount outstanding | 36,000,000 | | |
Unused borrowing capacity, amount | $ 90,500,000 | | |
Maximum borrowing capacity, percentage | 50.00% | | |
Secured Debt | Portfolio Loan | Revolving Credit Facility | Upon Meeting Certain Financial Coverage Ratio and Subject to Certain Conditions | | | |
Debt Instrument [Line Items] | | | |
Unused borrowing capacity, amount | $ 1,000,000 | | |
Secured Debt | Portfolio Loan | Non-Revolving Credit Facility | | | |
Debt Instrument [Line Items] | | | |
Maximum borrowing capacity, percentage | 50.00% | | |