SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($) yr $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares
Goodwill [Line Items]
Percentage Fair Value of Goodwill Exceeded Carrying Value	10.00%								10.00%
Value of Reporting Unit's Goodwill	$ 2,200.0								$ 2,200.0
Revenues	2,953.4	$ 2,928.5	$ 2,881.7	$ 2,791.2	$ 2,787.5	$ 2,831.3	$ 2,866.3	$ 2,848.3	11,554.8	$ 11,333.4	$ 10,308.0
Cash, Uninsured Amount	335.0				423.0				335.0	423.0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification									$ 1.6	10.2	16.9
Finite-Lived Intangible Asset, Useful Life									15 years
Cost of Revenue									$ 8,302.3	8,157.0	7,216.2
Gross profit	820.7	817.3	824.8	789.7	772.4	789.9	835.1	779.0	3,252.5	3,176.4	3,091.8
Selling, general and administrative expenses									1,624.5	1,570.9	1,499.2
Nonoperating Income (Expense)									(225.3)	(57.4)	(227.7)
Income Tax Expense (Benefit)									280.0	384.4	(155.4)
Net earnings									824.9	883.9	1,232.9
Net Income (Loss) Attributable to Noncontrolling Interest									(1.1)	(0.2)	(5.8)
Net earnings attributable to Laboratory Corporation of America Holdings	$ 227.1	$ 220.7	$ 190.4	$ 185.6	$ 157.9	$ 318.8	$ 233.8	$ 173.2	$ 823.8	$ 883.7	$ 1,227.1
Earnings Per Share, Basic | $ / shares	$ 2.34	$ 2.26	$ 1.94	$ 1.88	$ 1.58	$ 3.14	$ 2.29	$ 1.70	$ 8.42	$ 8.71	$ 11.99
Earnings Per Share, Diluted | $ / shares	$ 2.32	$ 2.25	$ 1.93	$ 1.86	$ 1.56	$ 3.10	$ 2.27	$ 1.67	$ 8.35	$ 8.61	$ 11.81
Net Cash Provided by (Used in) Operating Activities									$ 1,444.7	$ 1,305.4	$ 1,498.1
Prior Period Reclassification Adjustment										44.4
Net Cash Provided by (Used in) Investing Activities									(1,283.1)	206.7	(2,228.7)
Net Cash Provided by (Used in) Financing Activities									(252.7)	(1,389.9)	593.2
Effect of exchange rate changes on cash and cash equivalents									1.8	(12.0)	20.5
Cash and Cash Equivalents, Period Increase (Decrease)									$ (89.3)	110.2	(116.9)
Percentage Change in Discount Rate Used to Calculate Fair Value of Goodwill	1.00%								1.00%
Percentage Fair Value Exceeds Carrying Value with a 1% Change in Discount Rate Used to Calculate Fair Value of Goodwill	1.00%								1.00%
Greater Than Lease Term Months Required to Record Right of Use Asset and Lease Liability									12 years
Less Than Initial Lease Term Months For Short Term Lease Exemption and Recording on Balance Sheet									12 years
Goodwill	$ 7,865.0				$ 7,360.3				$ 7,865.0	7,360.3	7,400.9
Derivative, Fair Value, Net	0.0								0.0
Other Inventory, Supplies, Gross	228.3				200.1				228.3	200.1
Inventory, Finished Goods, Gross	16.4				37.2				$ 16.4	37.2
Ownership percentage below which investments are generally accounted for on the cost method (in hundredths)									20.00%
Accounts receivable balances (gross) from Medicare and Medicaid	$ 81.4				88.8				$ 81.4	88.8
Estimated useful life of capitalized software costs (in years) | yr									5
Minimum threshold percentage required to recognize income tax benefit (in hundredths)	50.00%								50.00%
Accounts receivable from Ontario government sponsored healthcare plan	$ 3.2				$ 0.5				$ 3.2	$ 0.5
Percent of gross accounts receivable due from patients	21.10%				21.50%				21.10%	21.50%
Minimum [Member] | Customer Relationships [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life									10 years
Minimum [Member] | Patents, Licenses And Technology [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life									3 years
Minimum [Member] | Non-compete agreements [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life									3 years
Minimum [Member] | Trade Names [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life									1 year
Maximum [Member] | Customer Relationships [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life									36 years
Maximum [Member] | Patents, Licenses And Technology [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life									15 years
Maximum [Member] | Non-compete agreements [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life									5 years
Maximum [Member] | Trade Names [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life									15 years
LabCorp Diagnostics [Member]
Goodwill [Line Items]
Revenues									$ 7,000.1	$ 7,030.8	6,858.2
Goodwill	$ 3,721.5				$ 3,638.8				3,721.5	3,638.8	3,673.9
Covance Drug Development [Member]
Goodwill [Line Items]
Revenues									4,578.1	4,313.1	3,451.6
Goodwill	$ 4,143.5				$ 3,721.5				$ 4,143.5	$ 3,721.5	$ 3,727.0