GOODWILL AND INTANGIBLE ASSETS (Changes in Carrying Amount of Goodwill) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Goodwill [Line Items]
Severance Costs	$ 30.0	$ 27.2
Intangible Assets, Gross (Excluding Goodwill)	5,545.2		$ 5,566.2
Balance as of January 1	7,400.9
Goodwill, Acquired During Period	49.5		1,009.8
Goodwill, Other Increase (Decrease)	0.0		170.5
Goodwill, Written off Related to Sale of Business Unit	(24.1)		0.0
Adjustments to goodwill	(63.6)		137.2
Balance at end of period	7,362.7		7,400.9
Finite-Lived Intangible Assets, Accumulated Amortization	1,554.3		1,400.1
Intangible Assets, Net (Excluding Goodwill)	3,990.9		4,166.1
LabCorp Diagnostics [Member]
Goodwill [Line Items]
Balance as of January 1	3,673.9
Goodwill, Acquired During Period	3.9		198.5
Goodwill, Other Increase (Decrease)	0.0		170.5
Goodwill, Written off Related to Sale of Business Unit	(24.1)		0.0
Adjustments to goodwill	(4.2)		1.1
Balance at end of period	3,649.5		3,673.9
Covance Drug Development [Member]
Goodwill [Line Items]
Balance as of January 1	3,727.0
Goodwill, Acquired During Period	45.6		811.3
Goodwill, Other Increase (Decrease)	0.0		0.0
Goodwill, Written off Related to Sale of Business Unit	0.0		0.0
Adjustments to goodwill	(59.4)		136.1
Balance at end of period	3,713.2		3,727.0
Customer Relationships [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	4,115.7		4,118.1
Finite-Lived Intangible Assets, Accumulated Amortization	1,106.5		992.8
Intangible Assets, Net (Excluding Goodwill)	3,009.2		3,125.3
Patents, Licenses And Technology [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	454.6		457.9
Finite-Lived Intangible Assets, Accumulated Amortization	207.8		188.6
Intangible Assets, Net (Excluding Goodwill)	246.8		269.3
Noncompete Agreements [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	75.8		75.8
Finite-Lived Intangible Assets, Accumulated Amortization	53.0		48.3
Intangible Assets, Net (Excluding Goodwill)	22.8		27.5
Trade Names [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	405.0		407.9
Finite-Lived Intangible Assets, Accumulated Amortization	183.3		167.9
Intangible Assets, Net (Excluding Goodwill)	221.7		240.0
Use Rights [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	10.8		10.8
Finite-Lived Intangible Assets, Accumulated Amortization	3.7		2.5
Intangible Assets, Net (Excluding Goodwill)	7.1		8.3
Canadian licenses [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	483.3		495.7
Finite-Lived Intangible Assets, Accumulated Amortization	0.0		0.0
Intangible Assets, Net (Excluding Goodwill)	$ 483.3		$ 495.7

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

Name:	us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

+ Details

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

Name:	us-gaap_GoodwillAcquiredDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_GoodwillLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase (decrease), classified as other, of an asset representing the future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

Name:	us-gaap_GoodwillOtherIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase (decrease) from foreign currency translation adjustments and purchase accounting adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

Name:	us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

Name:	us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount before accumulated amortization of intangible assets, excluding goodwill.

+ References

No definition available.

+ Details

Name:	us-gaap_IntangibleAssetsGrossExcludingGoodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph ((a)(1),(b))
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274

+ Details

Name:	us-gaap_IntangibleAssetsNetExcludingGoodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of expenses for special or contractual termination benefits provided to current employees involuntarily terminated under a benefit arrangement associated exit or disposal activities pursuant to an authorized plan. Excludes expenses related to one-time termination benefits, a discontinued operation or an asset retirement obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.P.3)
-URI http://asc.fasb.org/extlink&oid=115931487&loc=d3e140864-122747

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 5.P.4(b))
-URI http://asc.fasb.org/extlink&oid=115931487&loc=d3e140904-122747

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_SeveranceCosts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=lh_LabCorpDiagnosticsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=lh_CovanceDrugDevelopmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis=lh_PatentsLicensesAndTechnologyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis=us-gaap_NoncompeteAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis=us-gaap_TradeNamesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis=us-gaap_UseRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis=us-gaap_LicensingAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: