CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income [Member]
BALANCE at Dec. 31, 2017	$ 6,804.1	$ 12.0	$ 1,989.8	$ 6,196.1	$ (1,060.1)	$ (333.7)
Net earnings attributable to Laboratory Corporation of America Holdings	173.2			173.2
Other comprehensive earnings, net of tax	52.5					52.5
Issuance of common stock under employee stock plans	(28.4)	0.0	(28.4)
Surrender of restricted stock and performance share awards	(25.0)				(25.0)
Stock compensation	25.8		25.8
Purchase of common stock	(75.0)	0.0	75.0
BALANCE at Mar. 31, 2018	6,984.0	12.0	1,969.0	6,369.3	(1,085.1)	(281.2)
BALANCE at Dec. 31, 2017	6,804.1	12.0	1,989.8	6,196.1	(1,060.1)	(333.7)
Net earnings attributable to Laboratory Corporation of America Holdings	725.8
Other comprehensive earnings, net of tax	(74.1)
Conversion of zero-coupon convertible debt	0.3
BALANCE at Sep. 30, 2018	7,248.1	11.9	1,828.4	6,921.9	(1,106.3)	(407.8)
BALANCE at Mar. 31, 2018	6,984.0	12.0	1,969.0	6,369.3	(1,085.1)	(281.2)
Net earnings attributable to Laboratory Corporation of America Holdings	233.8			233.8
Other comprehensive earnings, net of tax	(125.8)					(125.8)
Issuance of common stock under employee stock plans	20.1		(14.6)
Issuance of common stock for acquisition consideration	14.6
Surrender of restricted stock and performance share awards					(20.1)
Conversion of zero-coupon convertible debt	26.2
Stock compensation	(75.0)		26.2
BALANCE at Jun. 30, 2018	7,037.7	12.0	1,934.8	6,603.1	(1,105.2)	(407.0)
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation			(75.0)
Net earnings attributable to Laboratory Corporation of America Holdings	318.8			318.8
Other comprehensive earnings, net of tax	(0.8)					(0.8)
Other Significant Noncash Transaction, Value of Consideration Given	24.4
Issuance of common stock under employee stock plans	(1.1)		(24.4)
Surrender of restricted stock and performance share awards					(1.1)
Conversion of zero-coupon convertible debt	0.3
Stock compensation	18.8		18.8
Purchase of common stock	(150.0)	0.1
BALANCE at Sep. 30, 2018	7,248.1	11.9	1,828.4	6,921.9	(1,106.3)	(407.8)
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation			(149.9)
BALANCE at Dec. 31, 2018	6,971.4	11.7	1,451.1	7,079.8	(1,108.1)	(463.1)
Net earnings attributable to Laboratory Corporation of America Holdings	185.6			185.6
Other comprehensive earnings, net of tax	23.6			0.0		23.6
Other Significant Noncash Transaction, Value of Consideration Given	24.7
Issuance of common stock under employee stock plans	(19.4)	0.0	(24.7)
Surrender of restricted stock and performance share awards					(19.4)
Stock compensation	25.5		25.5
Purchase of common stock	(100.1)	0.1	100.0
BALANCE at Mar. 31, 2019	7,111.3	11.6	1,401.3	7,265.4	(1,127.5)	(439.5)
BALANCE at Dec. 31, 2018	6,971.4	11.7	1,451.1	7,079.8	(1,108.1)	(463.1)
Net earnings attributable to Laboratory Corporation of America Holdings	596.7
Other comprehensive earnings, net of tax	(39.1)
Conversion of zero-coupon convertible debt	1.7
BALANCE at Sep. 30, 2019	7,231.1	9.0	47.8	7,676.5	0.0	(502.2)
BALANCE at Mar. 31, 2019	7,111.3	11.6	1,401.3	7,265.4	(1,127.5)	(439.5)
Net earnings attributable to Laboratory Corporation of America Holdings	190.4			190.4
Other comprehensive earnings, net of tax	27.6			0.0		27.6
Other Significant Noncash Transaction, Value of Consideration Given	9.2
Issuance of common stock under employee stock plans	(20.7)	0.0	(9.2)
Surrender of restricted stock and performance share awards					(20.7)
Treasury Stock, Retired, Cost Method, Amount	0.0	(2.4)	(1,145.8)		(1,148.2)
Stock compensation	26.5		26.5
Purchase of common stock	(199.9)	0.1
BALANCE at Jun. 30, 2019	7,144.4	9.1	91.4	7,455.8	0.0	(411.9)
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation			(199.8)
Net earnings attributable to Laboratory Corporation of America Holdings	220.7			220.7
Other comprehensive earnings, net of tax	(90.3)					(90.3)
Other Significant Noncash Transaction, Value of Consideration Given	25.1
Issuance of common stock under employee stock plans	(0.3)		(25.1)
Surrender of restricted stock and performance share awards		(0.3)
Stock compensation	31.5		31.5
Purchase of common stock	(100.0)	0.1	99.9
BALANCE at Sep. 30, 2019	$ 7,231.1	$ 9.0	$ 47.8	$ 7,676.5	$ 0.0	$ (502.2)