FAIR VALUE MEASUREMENTS (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 03, 2019 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Number of Measurement Funds Available For Participant Election	16
Noncontrolling interest puts	$ 15.2	$ 15.8
Fair market value of senior notes	6,433.0	6,140.6
Notes Receivable, Fair Value Disclosure	0.0		$ 110.0	$ 110.0
Cash Surrender Value, Fair Value Disclosure	78.6	80.2
Fair Value Liabilities Measured On Recurring Basis Deferred Compensation Liability	76.8	76.7
Contingent Consideration Classified as Equity, Fair Value Disclosure	9.0	9.9
Fair Value Hedges, Net	2.3	1.5
Foreign Currency Contract, Asset, Fair Value Disclosure		3.2
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	6.0
Contingent consideration adjustment	(2.1)
Equity Securities, FV-NI		9.1
lh_BusinessCombinationContingentConsiderationArrangementsPayments	(4.8)
Level 1 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Noncontrolling interest puts	0.0	0.0
Cash Surrender Value, Fair Value Disclosure	0.0	0.0
Fair Value Liabilities Measured On Recurring Basis Deferred Compensation Liability	0.0	0.0
Contingent Consideration Classified as Equity, Fair Value Disclosure	0.0	0.0
Fair Value Hedges, Net	0.0	0.0
Foreign Currency Contract, Asset, Fair Value Disclosure	0.0	0.0
Equity Securities, FV-NI		9.1
Fair Value, Inputs, Level 2 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Noncontrolling interest puts	15.2	15.8
Cash Surrender Value, Fair Value Disclosure	78.6	80.2
Fair Value Liabilities Measured On Recurring Basis Deferred Compensation Liability	76.8	76.7
Contingent Consideration Classified as Equity, Fair Value Disclosure	0.0	0.0
Fair Value Hedges, Net	2.3	1.5
Foreign Currency Contract, Asset, Fair Value Disclosure		3.2
Equity Securities, FV-NI		0.0
Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Noncontrolling interest puts	0.0	0.0
Cash Surrender Value, Fair Value Disclosure	0.0	0.0
Fair Value Liabilities Measured On Recurring Basis Deferred Compensation Liability	0.0	0.0
Contingent Consideration Classified as Equity, Fair Value Disclosure	9.0	9.9
Fair Value Hedges, Net	0.0	0.0
Foreign Currency Contract, Asset, Fair Value Disclosure	$ 0.0	0.0
Equity Securities, FV-NI		$ 0.0