INTANGIBLE ASSETS AND GOODWILL - Changes in Goodwill (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Goodwill balance before cumulative loss on impairment as of January 1,		$ 1,042,762	$ 1,024,467
Accumulated loss on impairment as of January 1,		(980,547)	$ (980,547)
Goodwill beginning balance after cumulative loss on impairment as of January 1,	$ 62,215	43,920
Acquisitions (See Note 2)	19,579	18,323
Measurement period adjustments to acquired goodwill (See Note 2)	(164)	(28)
Ending period balance	$ 81,630	$ 62,215