EARNINGS PER SHARE (Reconciliation of Basic Earnings Per Share to Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Income [Abstract]
Net earnings attributable to Laboratory Corporation of America Holdings	$ 703,400,000	$ 231,600,000	$ (317,200,000)	$ 220,700,000	$ 190,400,000	$ 185,600,000	$ 617,800,000	$ 596,700,000
Shares [Abstract]
Net earnings, basic (in shares)	97.4			97.6			97.3	98.1
Dilutive effect of employee stock options and awards, (in shares)	0.7			0.7			0.6	0.7
Per Share Amount [Abstract]
Basic earnings per common share (in dollars per share)	$ 7.22			$ 2.26			$ 6.35	$ 6.08
Diluted earnings per common share (in dollars per share)	$ 7.17			$ 2.25			$ 6.31	$ 6.04
Net Income (Loss) Available to Common Stockholders, Diluted	$ 703,400,000			$ 220,700,000			$ 617,800,000	$ 596,700,000
Weighted Average Number of Shares Outstanding, Diluted	98.1			98.3			97.9	98.8