AFFILIATE AND RELATED PARTY TRANSACTIONS - Amounts Due From and Due to Related Parties (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Treasury Stock, Shares, Acquired	6,066,251		6,629,415
Payments for Repurchase of Common Stock	$ 503,645	$ 726,005
Share based compensation expense	28,281	27,946
Related party expense	$ 2,001	2,642
Stock options \| 2017 LTIP
Related Party Transaction [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	409,478
Share based compensation expense	$ 24,223	20,349
Restricted Stock \| 2019 LTIP
Related Party Transaction [Line Items]
Share based compensation expense	317	576
CSC Holdings
Related Party Transaction [Line Items]
Cash distributions to parent	501,000	720,350
Intercompany settlement	745	(150,602)
Cash distributions	501,000	$ 720,350
Affiliates
Related Party Transaction [Line Items]
Due to related parties and affiliates	12,698		$ 8,538
Affiliates \| Other Related Party
Related Party Transaction [Line Items]
Due from related parties and affiliates	3,795		4,262
Due to related parties and affiliates	1,226		600
Affiliates \| Altice Management International
Related Party Transaction [Line Items]
Due to related parties and affiliates	$ 11,472		$ 7,938

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