RELATED PARTY TRANSACTIONS - Summary of Notes Payable to Related Party (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Notes payable	$ 0	$ 3,573	$ 12,700
Less: Current Portion	0	(3,573)	(5,000)
Notes payable to related party	0	0	7,700
Note payable to a related party due June 2021 [Member]
Debt Instrument [Line Items]
Notes payable	0	2,382	0
Note payable to a related party due May 2021 [Member]
Debt Instrument [Line Items]
Notes payable	$ 0	1,191	0
Notes payable converted into Senior Priority Units—Tranche 1 [Member]
Debt Instrument [Line Items]
Notes payable		$ 0	$ 12,700

The increase (decrease) during the reporting period in the current portion (due within one year or one business cycle, whichever is longer) of the amount owed by the reporting entity in the form of loans and obligations (generally evidenced by promissory notes) made by the following types of related parties: a parent company and its subsidiaries; subsidiaries of a common parent; an entity and trust for the benefit of employees, such as pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; an entity and its principal owners, management, or member of their immediate families; affiliates; or other parties with the ability to exert significant influence.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Name:	us-gaap_IncreaseDecreaseInNotesPayableRelatedPartiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration