SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill from business combinations
Goodwill	$ 25,111,000	$ 25,111,000
Capitalized costs	11,111,000	$ 10,490,000
Goodwill impairment charges	$ 0