DERIVATIVE CONTRACTS AND COLLATERALIZED INDEBTEDNESS - Location of Assets and Liabilities Within the Consolidated Balance Sheets (Details) - Not Designated as Hedging Instruments - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,555	$ 55,559
Liability Derivatives	525,002	568,803
Interest Rate Swap | Current derivative contracts
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,555	5,132
Interest Rate Swap | Long-term derivative contracts
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	4,774
Interest Rate Swap | Short-term liabilities under derivative contracts
Derivatives, Fair Value [Line Items]
Liability Derivatives	6,756	0
Interest Rate Swap | Long-term liabilities under derivative contracts
Derivatives, Fair Value [Line Items]
Liability Derivatives	(161,373)	(275,297)
Prepaid forward contracts | Current derivative contracts
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0	$ 45,653
Prepaid forward contracts | Short-term liabilities under derivative contracts
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	(45,653)
Prepaid forward contracts | Long-term liabilities under derivative contracts
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ (356,873)	$ (247,853)