FAIR VALUE MEASUREMENTS, Changes in Fair Value of Level 3 Warrant Liabilities (Details) - Private Placement [Member] - USD ($)	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021
Unobservable Input Reconciliation [Roll Forward]
Fair Value, beginning balance	$ 7,280,000	$ 6,400,000	$ 17,200,000	$ 7,840,000	$ 4,320,000	$ 11,270,000	$ 17,200,000
Change in fair value	(2,880,000)	880,000	(10,800,000)	1,200,000	3,520,000	400,000
Transfer to Level 1			0			(7,350,000)	0
Fair Value, ending balance	$ 4,400,000	$ 7,280,000	6,400,000	$ 9,040,000	$ 7,840,000	4,320,000	4,400,000
Transfer from Level 3			$ 0			$ (7,350,000)	0
Transfer to Level 3							$ 0