BASIS OF FINANCIAL STATEMENT PRESENTATION (Details) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues	$ 2,831.3	$ 2,621.4	$ 8,545.9	$ 7,563.3
Cost of Revenue	2,041.4	1,837.2	6,141.9	5,288.6
Contract with Customer, Liability	286.2		286.2			$ 377.4
Amortization of Deferred Sales Commissions	4.2	$ 3.5	12.8	10.6
Assets, Current	$ 3,274.3		3,274.3			2,733.8
Assets, Noncurrent						13,939.2
Net Cash Provided by (Used in) Operating Activities			$ 819.0	$ 933.1
Percent of Revenue Contributed	100.00%	100.00%	100.00%	100.00%
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification					$ 13.2
Selling, general and administrative expenses	$ 381.8	$ 381.3	$ 1,174.0	$ 1,081.9
Nonoperating Income (Expense)	156.2	(59.9)	35.8	(163.3)
Other Nonoperating Income		136.1		381.5
Provision for income taxes	180.6	92.5	328.1	268.6
Net earnings	319.0	174.3	725.7	542.7
Less: Net earnings attributable to the noncontrolling interest	(0.2)	(2.8)	0.1	(3.4)
Net Income (Loss) Attributable to Parent	$ 318.8	$ 171.5	$ 725.8	$ 539.3
Basic earnings per common share (in dollars per share) | $ / shares	$ 3.14	$ 1.68	$ 7.13	$ 5.27
Diluted earnings per common share (in dollars per share) | $ / shares	$ 3.10	$ 1.65	$ 7.04	$ 5.19
Net Cash Provided by (Used in) Investing Activities			$ 353.6	$ (2,050.4)
Net Cash Provided by (Used in) Financing Activities			(587.0)	1,073.5
Effect of exchange rate changes on cash and cash equivalents			(9.6)	19.3
Prior Period Reclassification Adjustment			45.1	46.5
Cash and Cash Equivalents, Period Increase (Decrease)			576.0	(24.5)
Assets	$ 16,661.4		16,661.4			16,673.0
Liabilities, Current	2,106.2		2,106.2			2,185.7
Liabilities, Noncurrent						7,662.4
Common stock, 92.8 and 93.5 shares outstanding at March 31, 2013 and December 31, 2012, respectively	20.4		20.4			20.8
Stockholders' Equity Attributable to Parent	7,248.1	$ 6,118.6	7,248.1	6,118.6	$ 5,518.2	6,804.1
Liabilities and Equity	16,661.4		16,661.4			16,673.0
Capitalized Contract Cost, Amortization	0.8	0.1	3.3	0.3
Deferred Revenue, Revenue Recognized			144.8
Revenue, Remaining Performance Obligation, Amount	4,199.6		4,199.6
Contract with Customer, Performance Obligation Satisfied in Previous Period	10.9		$ 20.5
Ownership percentage below which investments are generally accounted for on the cost method (in thousandths)			20.00%
LabCorp Diagnostics [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues	$ 1,752.0	$ 1,754.7	$ 5,336.3	$ 5,115.5
Percent of Revenue Contributed	62.00%	68.00%	62.00%	67.00%
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		$ 82.5		$ 238.7
Covance Drug Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues	$ 1,081.5	867.1	$ 3,214.1	2,448.7
Percent of Revenue Contributed	38.00%		38.00%
Investigator fees [Member] | Covance Drug Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		71.0		197.4
Measure of progress [Member] | Covance Drug Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		15.6		33.8
sales commissions [Member] | Covance Drug Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		2.0		0.8
Previous Accounting Guidance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues		2,655.2		7,645.1
Cost of Revenue		1,772.4		5,096.9
Net cash provided by (used in) operating activities, as reported				895.3
Current assets, as reported						2,682.6
Gross profit, as reported		882.8		2,548.2
Noncurrent assets, as reported						13,885.4
Total assets, as reported						16,568.0
Net Cash Provided by (Used in) Investing Activities, as reported				(2,050.4)
Selling, general and administrative expenses		465.3		1,320.0
Nonoperating Income (Expense)		136.4		382.3
Provision for income taxes		97.7		281.1
Net income (loss), including portion attributable to NCI, as reported		183.4		564.8
Less: Net earnings attributable to the noncontrolling interest		(2.8)		(3.4)
Net income (loss) attributable to Parent, as reported		$ 180.6		$ 561.4
Basic earnings per common share (in dollars per share) | $ / shares		$ 1.77		$ 5.48
Diluted earnings per common share (in dollars per share) | $ / shares		$ 1.74		$ 5.40
Net Cash Provided by (Used in) Financing Activities, as reported				$ 1,111.3
Effect of Exchange Rate on Cash and Cash Equivalents, as reported				19.4
Cash and Cash Equivalents, Period Increase (Decrease)				(24.4)
Current liabilities, as reported						2,046.1
Noncurrent liabilities, as reported						7,671.1
Noncontrolling Interest Mezzanine Equity, as reported						20.8
Shareholders Equity Attributable to Parent, as reported						6,830.0
Total liabilities and shareholders equity, as reported						$ 16,568.0
Other Current Assets [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				(51.2)
Other Noncurrent Assets [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				(53.8)
Assets [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				(105.0)
Other Current Liabilities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				(139.6)
Other Noncurrent Liabilities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				(8.7)
Noncontrolling Interest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				0.0
Shareholders' Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				(25.9)
Total liabilities and shareholder's equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				(105.0)
Revenue from Contract with Customer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts		$ (33.8)		(81.8)
ASU 715 pension adjustment to previously reported amounts		0.0		0.0
Gross profit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts		(98.4)		(273.1)
ASU 715 pension adjustment to previously reported amounts		(0.2)		(0.4)
Selling, General and Administrative Expenses [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts		(84.5)		(239.4)
ASU 715 pension adjustment to previously reported amounts		(0.5)		(1.3)
Nonoperating Income (Expense) [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts		(0.4)		(0.9)
ASU 715 pension adjustment to previously reported amounts		(0.7)		(1.7)
Provision for income taxes [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts		(5.2)		(12.5)
ASU 715 pension adjustment to previously reported amounts		0.0		0.0
Net earnings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts		(9.1)		(22.1)
ASU 715 pension adjustment to previously reported amounts		0.0		0.0
Net earnings attributable to NCI [Member]
Accounts, Notes, Loans and Financing Receivable [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
Cost of Sales [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts		(64.6)		(191.3)
ASU 715 pension adjustment to previously reported amounts		(0.2)		(0.4)
Net earnings attributable to company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts		(9.1)		(22.1)
ASU 715 pension adjustment to previously reported amounts		$ 0.0		0.0
Net cash from operating activities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				$ 0.0
ASC 230 Cash flow presentation adjustment				(8,700,000)
Prior Period Reclassification Adjustment				$ 46.5
Net cash from investing activities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				0.0
Net Cash Provided by (Used in) Investing Activities				0.0
Prior Period Reclassification Adjustment				0.0
Net cash from financing activities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				0.0
Net Cash Provided by (Used in) Financing Activities				8.7
Prior Period Reclassification Adjustment				(46.5)
Effect of exchange rate on cash and cash equivalents [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
ASU 606 adjustment to previously reported amounts				0.1
Effect of exchange rate changes on cash and cash equivalents				0.0
Prior Period Reclassification Adjustment				$ 0.0