BUSINESS ACQUISITIONS Business Acquisitions in the Aggregate (Details) - USD ($)		3 Months Ended			9 Months Ended		12 Months Ended
	Jun. 03, 2019	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Business Acquisition [Line Items]
Goodwill, Acquired During Period					$ 139,100,000
Business Acquisition, Pro Forma Revenue		$ 3,899,700,000		$ 2,969,000,000.0	9,524,200,000	$ 8,827,700,000
Revenues		3,896,100,000		2,928,500,000	9,488,700,000	8,601,400,000
Payments to Acquire Businesses, Net of Cash Acquired					208,800,000	852,900,000
Goodwill		7,613,800,000			7,613,800,000		$ 7,865,000,000.0
Business Acquisition, Pro Forma Net Income (Loss)		744,900,000		224,300,000	666,300,000	612,900,000
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability					6,000,000.0
Operating Income (Loss)		1,047,100,000		339,900,000	1,152,200,000	993,800,000
Notes Receivable, Fair Value Disclosure	$ 110,000,000.0	0		110,000,000.0	0	110,000,000.0
Total acquisition consideration (cash, stock, notes, etc.)	711,000,000.0		$ 711,000,000.0
Gain (Loss) on Disposition of Assets							12,200,000
Covance Drug Development [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period					73,000,000.0
Revenues		1,241,900,000		1,175,400,000	3,479,400,000	3,376,400,000
Goodwill		3,832,100,000			3,832,100,000		4,143,500,000
Operating Income (Loss)		142,000,000.0		123,800,000	(131,200,000)	277,600,000
LabCorp Diagnostics [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period					66,100,000
Revenues		2,704,200,000		1,759,200,000	6,098,800,000	5,242,100,000
Goodwill		3,781,700,000			3,781,700,000		$ 3,721,500,000
Operating Income (Loss)		964,900,000		$ 262,200,000	1,451,500,000	843,000,000.0
Other acquirees [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period					128,900,000	512,300,000
Revenues		13,100,000			13,800,000
Payments to Acquire Businesses, Net of Cash Acquired					208,800,000	852,900,000
Finite-lived Intangible Assets Acquired					88,500,000	324,100,000
Operating Income (Loss)		$ 3,500,000			3,600,000
Other acquirees [Member] | Covance Drug Development [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired					95,400,000	205,500,000
Other acquirees [Member] | LabCorp Diagnostics [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired					$ 113,400,000	$ 647,400,000
Other acquirees [Member] | Minimum [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life					5 years	11 years
Other acquirees [Member] | Maximum [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life					15 years	15 years
Envigo [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired	$ 601,000,000.0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			11 years
Notes Receivable, Fair Value Disclosure			$ 110,000,000.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			11,300,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables			12,100,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other			25,600,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory			4,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets			10,800,000
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets			128,400,000
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets			25,200,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles			141,400,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets			9,900,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			745,800,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable			15,200,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue			49,900,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities			144,800,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net			601,000,000.0
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net			376,600,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other			10,400,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities			69,300,000
Customer Relationships [Member] | Envigo [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets			140,800,000
Trade Names [Member] | Envigo [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles			$ 600,000