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
	Mar. 31, 2018	Mar. 31, 2017
Income [Abstract]
Net earnings attributable to Laboratory Corporation of America Holdings	$ 173.2	$ 183.0
Shares [Abstract]
Net earnings, basic (in shares)	101.9	102.5
Dilutive effect of employee stock options and awards, (in shares)	1.5	1.7
Effect of convertible debt, net of tax, (in shares)	0.0	0.1
Per Share Amount [Abstract]
Basic earnings per common share (in dollars per share)	$ 1.70	$ 1.79
Diluted earnings per common share (in dollars per share)	$ 1.67	$ 1.75
Net Income (Loss) Available to Common Stockholders, Diluted	$ 173.2	$ 183.0
Weighted Average Number of Shares Outstanding, Diluted	103.4	104.3