SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Real Estate Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Net amortization expense of acquired intangibles	$ 1,460	$ 4,506	$ 13,282
Future amortization expense, 2021	1,326
Future amortization expense, 2022	1,071
Future amortization expense, 2023	872
Future amortization expense, 2024	854
Future amortization expense, 2025	817
Interest expense capitalized	1,640	2,504	$ 3,225
Intangible Lease Assets And Other Assets | Above-market/Below-market leases
Finite Lived Intangible Assets [Line Items]
Intangible lease assets and liabilities, Cost	18,416	21,098
Intangible lease assets and liabilities, Accumulated Amortization	(16,395)	(18,559)
Intangible Lease Assets And Other Assets | In-place leases
Finite Lived Intangible Assets [Line Items]
Intangible lease assets and liabilities, Cost	59,472	66,309
Intangible lease assets and liabilities, Accumulated Amortization	(53,790)	(58,559)
Intangible Lease Assets And Other Assets | Tenant relationships
Finite Lived Intangible Assets [Line Items]
Intangible lease assets and liabilities, Cost	34,630	38,880
Intangible lease assets and liabilities, Accumulated Amortization	(7,909)	(10,834)
Accounts Payable and Accrued Liabilities | Above-market/Below-market leases
Finite Lived Intangible Assets [Line Items]
Intangible lease assets and liabilities, Cost	42,274	46,554
Intangible lease assets and liabilities, Accumulated Amortization	$ (36,224)	$ (38,052)
Buildings
Finite Lived Intangible Assets [Line Items]
Estimated useful life	40 years
Certain Improvements | Minimum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	10 years
Certain Improvements | Maximum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	20 years
Equipment and Fixtures | Minimum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	7 years
Equipment and Fixtures | Maximum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	10 years