QUARTERLY DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of unaudited quarterly data
Net sales	$ 2,701.5	$ 2,597.9	$ 2,498.4	$ 2,408.1	$ 2,387.3	$ 2,372.7	$ 2,382.0	$ 2,295.2	$ 10,205.9	$ 9,437.2	$ 8,505.7
Cost of Goods and Services Sold									6,741.9	6,256.7	5,602.4
Gross profit	915.8	882.8	861.8	803.6	788.0	788.4	826.8	777.3	3,464.0	3,180.5	2,903.3
Net earnings attributable to Laboratory Corporation of America Holdings	$ 706.8	$ 180.6	$ 188.6	$ 192.2	$ 184.4	$ 179.5	$ 204.1	$ 164.1	$ 1,268.2	$ 732.1	$ 437.6
Earnings Per Share
Basic earnings per share (in dollars per share)	$ 6.91	$ 1.77	$ 1.84	$ 1.87	$ 1.79	$ 1.74	$ 2.00	$ 1.61	$ 12.39	$ 7.14	$ 4.43
Diluted earnings per common share (in dollars per share)	$ 6.81	$ 1.74	$ 1.82	$ 1.84	$ 1.75	$ 1.71	$ 1.96	$ 1.58	$ 12.21	$ 7.02	$ 4.35