SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NET SALES (Details) - USD ($) $ 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
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
Intercompany revenue elimination									(1.8)	(0.8)	0.0
Revenues								9,641.8	10,441.4	9,641.8	8,680.1
Reimbursement Revenue								$ 204.6	$ 235.5	204.6	174.4
LabCorp Diagnostics [Member]
Percent of Revenue Contributed								69.90%	70.30%
Net sales									$ 7,170.5	6,593.9	6,199.3
Clinical diagnostics laboratory [Member]
Net sales									$ 7,170.5
Routine Testing [Member]
Net sales								$ 6,593.9			6,199.3
Genomic and Esoteric Testing [Member]
Net sales								$ 2,844.1			2,306.4
Covance Drug Development [Member]
Percent of Revenue Contributed								30.10%	29.70%
Net sales									$ 3,037.2	$ 2,844.1	2,306.4
Intercompany revenue elimination								$ (0.8)	(1.8)		$ 0.0
Other Segments [Member]
Net sales									$ 3,037.2