SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting standards update [Extensible Enumeration]	Accounting Standards Update 2016-02 [Member]
Prepaid expenses	$ (21,840)	$ (80,353)
Liabilities	(227,382)	$ (1,294,026)
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use assets	94,200
Lease, liability	126,000
Prepaid expenses	180
Liabilities	$ 31,800