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				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Income [Abstract]
Net earnings attributable to Laboratory Corporation of America Holdings	$ 231.6	$ (317.2)	$ 190.4	$ 185.6	$ (85.6)	$ 376.0
Shares [Abstract]
Net earnings, basic (in shares)	97.3		98.1		97.2	98.4
Dilutive effect of employee stock options and awards, (in shares)	0.4		0.7		0.0	0.7
Per Share Amount [Abstract]
Basic earnings per common share (in dollars per share)	$ 2.38		$ 1.94		$ (0.88)	$ 3.82
Diluted earnings per common share (in dollars per share)	$ 2.37		$ 1.93		$ (0.88)	$ 3.79
Net Income (Loss) Available to Common Stockholders, Diluted	$ 231.6		$ 190.4		$ (85.6)	$ 376.0
Weighted Average Number of Shares Outstanding, Diluted	97.7		98.8		97.2	99.1