INTANGIBLE ASSETS - Summary of Acquired Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 7,253,150	$ 7,190,428
Accumulated Amortization	(4,891,643)	(4,409,312)
Net Carrying Amount	2,361,507	2,781,116
Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,113,669	6,052,598
Accumulated Amortization	(3,885,700)	(3,478,742)
Net Carrying Amount	2,227,969	2,573,856
Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,081,083	1,081,083
Accumulated Amortization	(964,996)	(894,189)
Net Carrying Amount	116,087	186,894
Other amortizable intangibles
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	58,398	56,747
Accumulated Amortization	(40,947)	(36,381)
Net Carrying Amount	$ 17,451	$ 20,366
Minimum | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	3 years
Minimum | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	2 years
Minimum | Other amortizable intangibles
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	1 year
Maximum | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	18 years
Maximum | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	5 years
Maximum | Other amortizable intangibles
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	15 years