SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Millions	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Mar. 31, 2016 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) yr $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)	Dec. 31, 2012 USD ($)
Goodwill [Line Items]
Net cash provided by (used in) operating activities, as reported												$ 1,498.1	$ 1,197.1
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
Cash, Uninsured Amount	423.0				315.5						423.0	315.5
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											10.2	16.9	11.9	$ 33.5
Cost of Revenue											8,157.0	7,216.2	6,698.9
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
Selling, general and administrative expenses											1,570.9	1,499.2	1,345.5
Nonoperating Income (Expense)											(57.4)	(227.7)	(197.0)
Other Nonoperating Income												515.1	434.9
Income Tax Expense (Benefit)	384.4							(155.4)	$ 360.7		384.4	(155.4)	360.7
Net earnings											883.9	1,232.9	712.9
Net Income (Loss) Attributable to Noncontrolling Interest											(0.2)	(5.8)	(1.1)
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 | $ / shares	$ 1.58	$ 3.14	$ 2.29	$ 1.70	$ 6.73	$ 1.68	$ 1.80	$ 1.79			$ 8.71	$ 11.99	$ 6.94
Earnings Per Share, Diluted | $ / shares	$ 1.56	$ 3.10	$ 2.27	$ 1.67	$ 6.63	$ 1.65	$ 1.78	$ 1.75			$ 8.61	$ 11.81	$ 6.82
Net Cash Provided by (Used in) Operating Activities											$ 1,305.4	$ 1,498.1	$ 1,197.1
Prior Period Reclassification Adjustment												47.4	34.6
Net Cash Provided by (Used in) Investing Activities, as reported												(2,228.7)	(795.7)
Net Cash Provided by (Used in) Investing Activities											206.7	(2,228.7)	(795.7)
Net Cash Provided by (Used in) Financing Activities, as reported												593.2	(671.0)
Net Cash Provided by (Used in) Financing Activities											(1,389.9)	593.2	(671.0)
Effect of Exchange Rate on Cash and Cash Equivalents, as reported												20.5	(13.2)
Effect of exchange rate changes on cash and cash equivalents											(12.0)	20.5	(13.2)
Cash and Cash Equivalents, Period Increase (Decrease)											110.2	(116.9)	(282.8)
Assets, Current	$ 2,835.4				$ 2,733.8						2,835.4	2,733.8
Assets, Noncurrent					13,939.2							13,939.2
Assets	16,185.3				16,673.0						16,185.3	16,673.0
Liabilities, Current	1,878.9				2,185.7						1,878.9	2,185.7
Liabilities, Noncurrent					7,662.4							7,662.4
Noncontrolling interest	19.1				20.8						19.1	20.8
Stockholders' Equity Attributable to Parent	6,971.4				6,804.1						6,971.4	6,804.1	5,518.2		$ 4,945.1
Liabilities and Equity	16,185.3				16,673.0						16,185.3	16,673.0
Goodwill	7,360.3				7,400.9						7,360.3	7,400.9	6,424.4
Derivative, Fair Value, Net	0.0										0.0
Other Inventory, Supplies, Gross	200.1				195.2						200.1	195.2
Inventory, Finished Goods, Gross	37.2				32.4						37.2	32.4
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	$ 88.8				109.8						$ 88.8	109.8
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	$ 0.5				$ 12.9						$ 0.5	$ 12.9
Percent of gross accounts receivable due from patients	21.50%				20.90%						21.50%	20.90%
Assets Held-for-sale, Not Part of Disposal Group, Current, Other	$ 55.2										$ 55.2
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											5 years
Minimum [Member] | Trade Names [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life											5 years
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] | Trade Names [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life											15 years
LabCorp Diagnostics [Member]
Goodwill [Line Items]
Revenues											$ 7,030.8	$ 6,858.2	6,307.6
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification												312.5	287.9
Goodwill	3,638.8				$ 3,673.9						3,638.8	3,673.9	3,644.8
Covance Drug Development [Member]
Goodwill [Line Items]
Revenues											4,313.1	3,451.6	3,245.8
Goodwill	$ 3,721.5				3,727.0						$ 3,721.5	3,727.0	2,779.6
Revenue from Contract with Customer [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												(133.4)	115.7
ASU 715 pension adjustment to previously reported amounts												0.0	0.0
Cost of Sales [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												(238.6)	(435.8)
ASU 715 pension adjustment to previously reported amounts												(0.2)	(6.4)
Gross profit [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												(372.0)	(320.1)
ASU 715 pension adjustment to previously reported amounts												(0.2)	(6.4)
Selling, General and Administrative Expenses [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												(315.1)	(287.9)
ASU 715 pension adjustment to previously reported amounts												(1.9)	(3.2)
Nonoperating Income (Expense) [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												(0.5)	0.3
ASU 715 pension adjustment to previously reported amounts												(2.1)	(9.6)
Provision for income taxes [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												(16.3)	(11.6)
ASU 715 pension adjustment to previously reported amounts												0.0	0.0
Net earnings [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												(41.1)	(20.3)
ASU 715 pension adjustment to previously reported amounts												0.0	0.0
Net earnings attributable to NCI [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												0.0	0.0
ASU 715 pension adjustment to previously reported amounts												0.0	0.0
Net earnings attributable to company [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts												(41.1)	(20.3)
ASU 715 pension adjustment to previously reported amounts												0.0	0.0
Net cash from operating activities [Member]
Goodwill [Line Items]
Prior Period Reclassification Adjustment												$ 47.4	$ 34.6
ASC 230 Cash flow presentation adjustment												(8,700,000)	(13,400,000)
Net cash from investing activities [Member]
Goodwill [Line Items]
Prior Period Reclassification Adjustment												$ 0.0	$ 0.0
Net Cash Provided by (Used in) Investing Activities												0.0	0.0
Net cash from financing activities [Member]
Goodwill [Line Items]
Prior Period Reclassification Adjustment												(47.4)	(34.6)
Net Cash Provided by (Used in) Financing Activities												8.7	13.4
Effect of exchange rate on cash and cash equivalents [Member]
Goodwill [Line Items]
Prior Period Reclassification Adjustment												0.0	0.0
Effect of exchange rate changes on cash and cash equivalents												0.0	0.0
Other Current Assets [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts										$ (51.2)
Other Noncurrent Assets [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts										(53.8)
Assets [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts										(105.0)
Other Current Liabilities [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts										(139.6)
Other Noncurrent Liabilities [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts										(8.7)
Noncontrolling Interest [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts										0.0
Shareholders' Equity [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts										(25.9)
Total liabilities and shareholder's equity [Member]
Goodwill [Line Items]
ASU 606 adjustment to previously reported amounts										$ (105.0)
Previously Reported [Member]
Goodwill [Line Items]
Current assets, as reported					2,682.6							2,682.6
Net cash provided by (used in) operating activities, as reported												1,459.4	1,175.9
Revenues												10,441.4	9,437.2
Cost of Revenue												6,977.4	6,256.7
Gross profit, as reported												3,464.0	3,180.5
Selling, general and administrative expenses												1,812.4	1,630.2
Nonoperating Income (Expense)												516.7	444.8
Income Tax Expense (Benefit)												(139.1)	372.3
Net income (loss), including portion attributable to NCI, as reported												1,274.0	733.2
Net Income (Loss) Attributable to Noncontrolling Interest												(5.8)	(1.1)
Net income (loss) attributable to Parent, as reported												$ 1,268.2	$ 732.1
Earnings Per Share, Basic | $ / shares												$ 12.39	$ 7.14
Earnings Per Share, Diluted | $ / shares												$ 12.21	$ 7.02
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)
Noncurrent assets, as reported					13,885.4							13,885.4
Total assets, as reported					16,568.0							16,568.0
Current liabilities, as reported					2,046.1							2,046.1
Noncurrent liabilities, as reported					7,671.1							7,671.1
Noncontrolling Interest Mezzanine Equity, as reported					20.8							20.8
Shareholders Equity Attributable to Parent, as reported					6,830.0							6,830.0
Total liabilities and shareholders equity, as reported					$ 16,568.0							16,568.0
Investigator fees [Member] | Covance Drug Development [Member]
Goodwill [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification												267.6	234.5
Measure of progress [Member] | Covance Drug Development [Member]
Goodwill [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification												58.9	36.1
sales commissions [Member] | Covance Drug Development [Member]
Goodwill [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification												$ 2.9	$ 4.4