RESTRUCTURING AND OTHER SPECIAL CHARGES (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Goodwill, Acquired During Period	$ 49.5		$ 1,009.8
Restructuring Reserve	48.1		54.7
Restructuring Charges	38.8
Restructuring Reserve Settled With Cash And Other Adjustment	(43.1)
Net restructuring charges	36.5	$ 64.6
Restructuring charges related to severance and other employee costs	30.0	27.2
Restructuring charges related to contractual obligations associated with leased facilities and other facility related costs	8.8	18.0
Employee Severance Benefits Related Restructuring Reserve Accrual Adjustment	1.1	1.4
Asset Impairment Charges	5.3	23.5
Unused facility restructuring reserves	1.2	0.1
Reduction in total prior restructuring accruals	(2.3)
Restructuring Reserve, Current	22.6
Restructuring Reserve, Noncurrent	25.5
special charge - consulting expense	4.3	8.0
Business Combination, Acquisition Related Costs		29.8
Special charge, accelerated equity compensation		0.9
Special charge, accelerated equity compensation	43.1
Special charge, malware attack	12.6
Special charge - one-time tax liability	31.1
Covance Drug Development [Member]
Restructuring Cost and Reserve [Line Items]
Goodwill, Acquired During Period	45.6		811.3
Net restructuring charges	23.3	50.6
Asset Impairment Charges	5.3	20.9
Covance Drug Development [Member] | Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	10.1		8.3
Restructuring Charges	19.3
Restructuring Reserve Settled With Cash And Other Adjustment	(16.3)
Reduction in total prior restructuring accruals	1.2
Covance Drug Development [Member] | Facility Closing [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	29.4		34.6
Restructuring Charges	5.8
Restructuring Reserve Settled With Cash And Other Adjustment	(10.4)
Reduction in total prior restructuring accruals	0.6
LabCorp Diagnostics [Member]
Restructuring Cost and Reserve [Line Items]
Goodwill, Acquired During Period	3.9		198.5
Net restructuring charges	13.2	14.0
LabCorp Diagnostics [Member] | Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	1.3		1.7
Restructuring Charges	10.7
Restructuring Reserve Settled With Cash And Other Adjustment	(11.1)
Reduction in total prior restructuring accruals	0.0
LabCorp Diagnostics [Member] | Facility Closing [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	7.3		$ 10.1
Restructuring Charges	3.0
Restructuring Reserve Settled With Cash And Other Adjustment	(5.3)
Reduction in total prior restructuring accruals	(0.5)
Selling, General and Administrative Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Other Special Charges	$ 7.3	$ 8.2