SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NET SALES (Details) - USD ($) $ 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	Dec. 31, 2015
Net cash provided by (used in) operating activities, as reported										$ 1,498.1	$ 1,197.1
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification									$ 10.2	$ 16.9	11.9	$ 33.5
Percent of Revenue Contributed									100.00%	100.00%
Intercompany revenue elimination									$ (10.5)	$ (1.8)	(1.0)
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
Net Cash Provided by (Used in) Investing Activities, as reported										(2,228.7)	(795.7)
Net Cash Provided by (Used in) Financing Activities, as reported										593.2	(671.0)
Effect of Exchange Rate on Cash and Cash Equivalents, as reported										20.5	(13.2)
Cash and Cash Equivalents, Period Increase (Decrease)									$ 110.2	(116.9)	(282.8)
LabCorp Diagnostics [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification										$ 312.5	287.9
Percent of Revenue Contributed								66.50%	62.00%	67.00%
Revenues									$ 7,030.8	$ 6,858.2	6,307.6
Covance Drug Development [Member]
Percent of Revenue Contributed								33.50%	38.10%
Revenues									$ 4,313.1	3,451.6	3,245.8
Previously Reported [Member]
Net cash provided by (used in) operating activities, as reported										1,459.4	1,175.9
Revenues										10,441.4	9,437.2
Net Cash Provided by (Used in) Investing Activities, as reported										(2,228.7)	(795.7)
Net Cash Provided by (Used in) Financing Activities, as reported										631.9	(649.8)
Effect of Exchange Rate on Cash and Cash Equivalents, as reported										20.5	(13.2)
Cash and Cash Equivalents, Period Increase (Decrease)										(116.9)	(282.8)
Investigator fees [Member] | Covance Drug Development [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification										267.6	234.5
Measure of progress [Member] | Covance Drug Development [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification										58.9	36.1
sales commissions [Member] | Covance Drug Development [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification										$ 2.9	$ 4.4