SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Restructuring Costs (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restructuring Reserve [Roll Forward]
Charged to restructuring costs	$ 318			$ 2,316	$ 2,391	$ 5,210
Restructuring costs	318			$ 2,316	2,391	$ 5,210
Employee Termination Costs
Restructuring Reserve [Roll Forward]
Balance at beginning of period	594	$ 979	$ 499		499
Charged to restructuring costs	(10)	2	824
Payments	(278)	(387)	(344)
Balance at end of period	306	594	979		306
Restructuring costs	(10)	2	824
Lease-Related Exit Costs
Restructuring Reserve [Roll Forward]
Balance at beginning of period	2,780	2,526	2,687		2,687
Charged to restructuring costs	47	458	46
Payments	(194)	(204)	(207)
Balance at end of period	2,633	2,780	2,526		2,633
Restructuring costs	$ 47	$ 458	$ 46
Strategic Reduction of Real Estate Footprint
Restructuring Reserve [Roll Forward]
Charged to restructuring costs					1,000
Restructuring costs					$ 1,000