QUARTERLY DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of unaudited quarterly data
Revenues	$ 2,787.5	$ 2,831.3	$ 2,866.3	$ 2,848.3	$ 2,744.7	$ 2,621.4	$ 2,528.2	$ 2,413.7	$ 11,333.4	$ 10,308.0	$ 9,552.9
Gross profit	772.4	789.9	835.1	779.0	817.1	784.2	778.0	712.5	3,176.4	3,091.8	2,854.0
Operating Income (Loss)	307.7	343.4	369.2	305.4	330.6	326.7	329.8	318.1	1,325.7	1,305.2	1,270.6
Net earnings attributable to Laboratory Corporation of America Holdings	$ 157.9	$ 318.8	$ 233.8	$ 173.2	$ 687.8	$ 171.5	$ 184.8	$ 183.0	$ 883.7	$ 1,227.1	$ 711.8
Earnings Per Share
Basic earnings per share (in dollars per share)	$ 1.58	$ 3.14	$ 2.29	$ 1.70	$ 6.73	$ 1.68	$ 1.80	$ 1.79	$ 8.71	$ 11.99	$ 6.94
Diluted earnings per common share (in dollars per share)	$ 1.56	$ 3.10	$ 2.27	$ 1.67	$ 6.63	$ 1.65	$ 1.78	$ 1.75	$ 8.61	$ 11.81	$ 6.82