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SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE (Details) |  |  |  |  |  |  |  |  |  |  |  | 12 Months Ended |  |
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Dec. 27, 2021 USD ($) | Jul. 01, 2021 USD ($) | Jun. 17, 2021 USD ($) | Dec. 15, 2020 USD ($) | Jul. 01, 2020 USD ($) | Feb. 11, 2020 USD ($) | Dec. 10, 2019 USD ($) | Oct. 21, 2019 USD ($) | Aug. 29, 2019 USD ($) | Aug. 23, 2019 USD ($) | Jun. 20, 2019 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Jan. 31, 2022 USD ($) property | Sep. 30, 2021 USD ($)
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayment costs on debt related to real estate |  |  |  |  |  |  |  |  |  |  |  | $ 901,000 | $ 1,123,000 | $ 827,000 |  |
Write-off of unbilled rent receivable, as a reduction to Gain on sale of real estate, net |  |  |  |  |  |  |  |  |  |  |  | 1,438,000 | 365,000 | 182,000 |  |
Property held-for-sale |  |  |  |  |  |  |  |  |  |  |  | 1,270,000 |  |  |  |
Gain on sale of real estate, net |  |  |  |  |  |  |  |  |  |  |  | 25,463,000 | 17,280,000 | 4,327,000 |  |
Pet Smart and Guitar Center retail properties, Houston, Texas | Other receivables |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Seller-financing provided by the Company |  |  |  |  |  |  |  |  |  |  |  |  | 4,613,000 |  |  |
Properties disposed of by sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  |  |  |  |  |  |  |  |  | 54,510,000 | 34,340,000 | 41,891,000 |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  |  |  |  |  |  |  |  |  | 25,463,000 | 17,280,000 | 4,327,000 |  |
Mortgage Prepaid on Sale |  |  |  |  |  |  |  |  |  |  |  | 20,387,000 | 11,815,000 | 17,507,000 |  |
Prepayment costs on debt related to real estate |  |  |  |  |  |  |  |  |  |  |  | 848,000 | 1,123,000 | 827,000 |  |
Write-off of unbilled rent receivable, as a reduction to Gain on sale of real estate, net |  |  |  |  |  |  |  |  |  |  |  | 1,438,000 | 365,000 | 182,000 |  |
Write-off of unamortized lease assets, as a reduction to Gain on sale of real estate, net |  |  |  |  |  |  |  |  |  |  |  | 967,000 | $ 367,000 | $ 915,000 |  |
Properties disposed of by sale | Whole Foods retail property & parking lot, West Hartford, Connecticut |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  | $ 40,510,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  | 21,469,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Prepaid on Sale |  |  | 15,403,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayment costs on debt related to real estate |  |  | $ 799,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Properties disposed of by sale | Vacant retail property, Philadelphia, Pennsylvania |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  | $ 8,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  | 1,299,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Prepaid on Sale |  | 3,574,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayment costs on debt related to real estate |  | $ 26,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Properties disposed of by sale | Wendys restaurant property, Hanover, Pennsylvania |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price | $ 2,815,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net | 1,331,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Prepaid on Sale | 696,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayment costs on debt related to real estate | 11,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Properties disposed of by sale | Wendys restaurant property, Gettysburg, Pennsylvania |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price | 2,885,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net | 1,364,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Prepaid on Sale | 714,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayment costs on debt related to real estate | $ 12,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Properties disposed of by sale | Hobby Lobby retail property, Onalaska, Wisconsin |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  |  |  | $ 7,115,000 |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  |  |  | 4,252,000 |  |  |  |  |  |  |  |  |  |
Mortgage Prepaid on Sale |  |  |  |  |  | 3,332,000 |  |  |  |  |  |  |  |  |  |
Prepayment costs on debt related to real estate |  |  |  |  |  | $ 290,000 |  |  |  |  |  |  |  |  |  |
Properties disposed of by sale | CarMax retail property, Knoxville, Tennessee |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  |  | $ 18,000,000 |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  |  | 10,316,000 |  |  |  |  |  |  |  |  |  |  |
Mortgage Prepaid on Sale |  |  |  |  | 8,483,000 |  |  |  |  |  |  |  |  |  |  |
Prepayment costs on debt related to real estate |  |  |  |  | $ 833,000 |  |  |  |  |  |  |  |  |  |  |
Properties disposed of by sale | Pet Smart retail property, Houston, Texas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  | $ 4,013,000 |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  | 1,067,000 |  |  |  |  |  |  |  |  |  |  |  |
Properties disposed of by sale | Guitar Center retail property, Houston, Texas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  | 5,212,000 |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  | $ 1,645,000 |  |  |  |  |  |  |  |  |  |  |  |
Properties disposed of by sale | Kmart retail property, Clemmons, North Carolina |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  |  |  |  |  |  |  |  | $ 5,500,000 |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  |  |  |  |  |  |  |  | 1,099,000 |  |  |  |  |
Mortgage Prepaid on Sale |  |  |  |  |  |  |  |  |  |  | 1,705,000 |  |  |  |  |
Prepayment costs on debt related to real estate |  |  |  |  |  |  |  |  |  |  | $ 41,000 |  |  |  |  |
Properties disposed of by sale | Multi-tenant retail property, Athens, Georgia |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  |  |  |  |  |  |  | $ 6,050,000 |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  |  |  |  |  |  |  | 1,045,000 |  |  |  |  |  |
Mortgage Prepaid on Sale |  |  |  |  |  |  |  |  |  | 2,645,000 |  |  |  |  |  |
Prepayment costs on debt related to real estate |  |  |  |  |  |  |  |  |  | $ 161,000 |  |  |  |  |  |
Properties disposed of by sale | Land - The Briarbrook Village Apartments, Wheaton, Illinois |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  |  |  |  |  |  | $ 12,066,000 |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  |  |  |  |  |  | $ 1,530,000 |  |  |  |  |  |  |
Properties disposed of by sale | Aaron's retail property, Houston, Texas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  |  |  |  |  | $ 1,675,000 |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  |  |  |  |  | $ 218,000 |  |  |  |  |  |  |  |
Properties disposed of by sale | Assisted living facility, Round Rock, Texas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Sales Price |  |  |  |  |  |  | $ 16,600,000 |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Real Estate, Net |  |  |  |  |  |  | 435,000 |  |  |  |  |  |  |  |  |
Mortgage Prepaid on Sale |  |  |  |  |  |  | 13,157,000 |  |  |  |  |  |  |  |  |
Prepayment costs on debt related to real estate |  |  |  |  |  |  | $ 625,000 |  |  |  |  |  |  |  |  |
Properties held-for-sale | Industrial property located in Columbus, Ohio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contracted sale price of property |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,500,000
Property held-for-sale |  |  |  |  |  |  |  |  |  |  |  | $ 1,270,000 |  |  |  |
Properties held-for-sale | Four restaurant properties located in Pennsylvania |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties held-for-sale | property |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |
Contracted sale price of property |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10,000,000 |
Consolidated Joint Venture-VIEs | Properties disposed of by sale | Vacant retail property, Philadelphia, Pennsylvania |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest in consolidated joint venture of the company (as a percent) |  | 90.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-controlling interest's share of the gain |  | $ 130,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Consolidated Joint Venture-VIEs | Properties disposed of by sale | Kmart retail property, Clemmons, North Carolina |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SALES OF PROPERTIES AND PROPERTY HELD-FOR-SALE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest in consolidated joint venture of the company (as a percent) |  |  |  |  |  |  |  |  |  |  | 90.00% |  |  |  |  |
Non-controlling interest's share of the gain |  |  |  |  |  |  |  |  |  |  | $ 422,000 |  |  |  |  |

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