FAIR VALUE MEASUREMENT - Purchase Option Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Movement In Purchase Option Value [Roll Forward]
Loss on impairment and write-off of assets	$ (1,759)	$ (2,521)	$ (1,075)
Real Estate Loan 1
Movement In Purchase Option Value [Roll Forward]
Purchase Option value, beginning balance	2,382
Loss on impairment and write-off of assets	(782)
Purchase Option value, ending balance	1,600	2,382
Real Estate Loan 2
Movement In Purchase Option Value [Roll Forward]
Purchase Option value, beginning balance	2,761
Loss on impairment and write-off of assets	0
Purchase Option value, ending balance	2,761	2,761
Real Estate Loan 3
Movement In Purchase Option Value [Roll Forward]
Purchase Option value, beginning balance	977
Loss on impairment and write-off of assets	(977)
Purchase Option value, ending balance	0	977
Real Estate Loan 4
Movement In Purchase Option Value [Roll Forward]
Purchase Option value, beginning balance	2,800
Loss on impairment and write-off of assets	0
Purchase Option value, ending balance	$ 2,800	$ 2,800