AFFILIATE AND RELATED PARTY TRANSACTIONS - Amounts Due From and Due to Related Parties (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Treasury Stock, Shares, Acquired							21,748		6,629,415
Payments for Repurchase of Common Stock							$ 804,928	$ 1,814,689
Share based compensation expense	$ 24,350			$ 34,710			80,277	96,974
Related party expense	5,291			1,846			$ 11,457	5,008
Stock options | 2017 LTIP
Related Party Transaction [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross							1,102,435
Share based compensation expense	21,886			27,601			$ 70,582	73,277
Restricted Stock | 2019 LTIP
Related Party Transaction [Line Items]
Share based compensation expense	178			653			756	1,864
CSC Holdings
Related Party Transaction [Line Items]
Cash distributions to parent	(77,957)	$ (184,478)	$ (501,000)	(431,100)	$ (652,383)	$ (720,350)	(763,435)	(1,803,833)
Intercompany settlement				52,421	35,415
Cash distributions							763,435	1,803,833
Non-cash distributions to parent	(754)	(19,660)	(748)			(150,602)
CSC Holdings | Member's Deficiency
Related Party Transaction [Line Items]
Cash distributions to parent	(77,957)	(184,478)	(501,000)	(431,100)	(652,383)	(720,350)
Intercompany settlement				$ 52,421	$ 35,415
Non-cash distributions to parent	(754)	$ (19,660)	$ (748)			$ (150,602)	(21,162)	$ (62,766)
Affiliates
Related Party Transaction [Line Items]
Due to related parties and affiliates	20,972						20,972		$ 8,538
Affiliates | Other Related Party
Related Party Transaction [Line Items]
Due from related parties and affiliates	3,892						3,892		4,262
Due to related parties and affiliates	1,030						1,030		600
Affiliates | Altice Management International
Related Party Transaction [Line Items]
Due to related parties and affiliates	$ 19,942						$ 19,942		$ 7,938