EARNINGS PER SHARE (Reconciliation of Basic Earnings Per Share to Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Income [Abstract]
Net earnings attributable to Laboratory Corporation of America Holdings	$ 220.7	$ 190.4	$ 185.6	$ 318.8	$ 233.8	$ 173.2	$ 596.7	$ 725.8
Shares [Abstract]
Net earnings, basic (in shares)	97.6			101.6			98.1	101.8
Dilutive effect of employee stock options and awards, (in shares)	0.7			1.0			0.7	1.2
Per Share Amount [Abstract]
Basic earnings per common share (in dollars per share)	$ 2.26			$ 3.14			$ 6.08	$ 7.13
Diluted earnings per common share (in dollars per share)	$ 2.25			$ 3.10			$ 6.04	$ 7.04
Net Income (Loss) Available to Common Stockholders, Diluted	$ 220.7			$ 318.8			$ 596.7	$ 725.8
Weighted Average Number of Shares Outstanding, Diluted	98.3			102.7			98.8	103.1