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, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Income [Abstract]
Net earnings attributable to Laboratory Corporation of America Holdings	$ 190.4	$ 185.6	$ 233.8	$ 173.2	$ 376.0	$ 407.0
Shares [Abstract]
Net earnings, basic (in shares)	98.1		101.9		98.4	101.9
Dilutive effect of employee stock options and awards, (in shares)	0.7		1.0		0.7	1.3
Per Share Amount [Abstract]
Basic earnings per common share (in dollars per share)	$ 1.94		$ 2.29		$ 3.82	$ 3.99
Diluted earnings per common share (in dollars per share)	$ 1.93		$ 2.27		$ 3.79	$ 3.94
Net Income (Loss) Available to Common Stockholders, Diluted	$ 190.4		$ 233.8		$ 376.0	$ 407.0
Weighted Average Number of Shares Outstanding, Diluted	98.8		103.0		99.1	103.3