GOODWILL AND INTANGIBLE ASSETS (Changes in Carrying Amount of Goodwill) (Details) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Goodwill [Line Items]
Amortization of intangibles and other assets	$ 62,200,000		$ 61,700,000	$ 184,600,000	$ 179,000,000.0
Severance Costs				12,800,000		$ 26,200,000
Intangible Assets, Gross (Excluding Goodwill)	5,953,100,000			5,953,100,000		5,885,600,000
Balance as of January 1		$ 7,865,000,000.0		7,865,000,000.0
Adjustments to goodwill				32,100,000
Balance at end of period	7,613,800,000			7,613,800,000		7,865,000,000.0
Finite-Lived Intangible Assets, Accumulated Amortization	2,032,600,000			2,032,600,000		1,851,100,000
Intangible Assets, Net (Excluding Goodwill)	3,920,500,000			3,920,500,000		4,034,500,000
Goodwill, Impairment Loss				422,400,000
Goodwill, Acquired During Period				139,100,000
LabCorp Diagnostics [Member]
Goodwill [Line Items]
Balance as of January 1		$ 3,721,500,000		3,721,500,000
Adjustments to goodwill				(2,200,000)
Balance at end of period	3,781,700,000			3,781,700,000		3,721,500,000
Goodwill, Impairment Loss				3,700,000
Percentage Fair Value of Goodwill Exceeded Carrying Value		10.00%
Percentage Change in Discount Rate Used to Calculate Fair Value of Goodwill		1.00%
Value of Goodwill Impairment Following a Percentage Change in the Discount Rate Used to Calculate Goodwill for Reporting Unit		$ 40,000,000.0
Impairment of Intangible Assets (Excluding Goodwill)				30,500,000
Value of Reporting Units Goodwill Fair Value Exceeds Carrying Value	86.0			86.0
Goodwill, Acquired During Period				66,100,000
Covance Drug Development [Member]
Goodwill [Line Items]
Balance as of January 1		$ 4,143,500,000		4,143,500,000
Adjustments to goodwill				34,300,000
Balance at end of period	3,832,100,000			3,832,100,000		4,143,500,000
Goodwill, Impairment Loss				418,700,000
Percentage Fair Value of Goodwill Exceeded Carrying Value		10.00%
Percent Change in Discount Rate Used to Calculate Fair Value of Goodwill for Reporting Unit		1.00%
Percent of Fair Value Exceeding Carrying Value Post Percent Change in Discount Rate Used to Calculate Fair Value of Goodwill for Reporting Unit		2.00%
Value of Reporting Unit's Goodwill Following Impairment		$ 1,627,500,000
Value of Reporting Units Goodwill Fair Value Exceeds Carrying Value	646,200,000			646,200,000
Goodwill, Acquired During Period				73,000,000.0
Customer Relationships [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	4,530,700,000			4,530,700,000		4,441,700,000
Finite-Lived Intangible Assets, Accumulated Amortization	1,473,100,000			1,473,100,000		1,329,500,000
Intangible Assets, Net (Excluding Goodwill)	3,057,600,000			3,057,600,000		3,112,200,000
Patents, Licenses And Technology [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	427,700,000			427,700,000		453,600,000
Finite-Lived Intangible Assets, Accumulated Amortization	245,300,000			245,300,000		235,700,000
Intangible Assets, Net (Excluding Goodwill)	182,400,000			182,400,000		217,900,000
Noncompete Agreements [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	105,300,000			105,300,000		90,900,000
Finite-Lived Intangible Assets, Accumulated Amortization	67,600,000			67,600,000		60,500,000
Intangible Assets, Net (Excluding Goodwill)	37,700,000			37,700,000		30,400,000
Trade Names [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	410,200,000			410,200,000		408,200,000
Finite-Lived Intangible Assets, Accumulated Amortization	240,100,000			240,100,000		219,900,000
Intangible Assets, Net (Excluding Goodwill)	170,100,000			170,100,000		188,300,000
Use Rights [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	10,800,000			10,800,000		10,900,000
Finite-Lived Intangible Assets, Accumulated Amortization	6,500,000			6,500,000		5,500,000
Intangible Assets, Net (Excluding Goodwill)	4,300,000			4,300,000		5,400,000
Canadian licenses [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	468,400,000			468,400,000		480,300,000
Finite-Lived Intangible Assets, Accumulated Amortization	0			0		0
Intangible Assets, Net (Excluding Goodwill)	$ 468,400,000			$ 468,400,000		$ 480,300,000
Trade Names [Member] | Covance Drug Development [Member]
Goodwill [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)		$ 2,700,000