BASIS OF FINANCIAL STATEMENT PRESENTATION (Details) - USD ($)	1 Months Ended	3 Months Ended						9 Months Ended
	Apr. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Asset Impairment Charges		$ 23,500,000			$ 0			$ 460,900,000	$ 0
Revenues		3,896,100,000			2,928,500,000			9,488,700,000	8,601,400,000
Cost of Revenue		2,336,700,000			2,111,200,000			6,440,800,000	6,169,600,000
Contract with Customer, Liability		492,900,000						492,900,000		$ 449,200,000
Amortization of Deferred Sales Commissions		6,000,000.0			$ 5,800,000			16,800,000	15,300,000
Assets, Current		$ 4,085,600,000						4,085,600,000		2,981,200,000
Net Cash Provided by (Used in) Operating Activities								$ 1,360,700,000	$ 874,900,000
Percent of Revenue Contributed		100.00%			100.00%			100.00%	100.00%
Selling, general and administrative expenses		$ 419,500,000			$ 401,500,000			$ 1,211,300,000	$ 1,210,600,000
Nonoperating Income (Expense)		(100,000,000.0)			(52,500,000)			(175,700,000)	(181,800,000)
Provision for income taxes		243,400,000			66,400,000			358,000,000.0	214,400,000
Net earnings		703,700,000			221,000,000.0			618,400,000	597,600,000
Net Income (Loss) Attributable to Parent		$ 703,400,000	$ 231,600,000	$ (317,200,000)	$ 220,700,000	$ 190,400,000	$ 185,600,000	$ 617,800,000	$ 596,700,000
Basic earnings per common share (in dollars per share)		$ 7.22			$ 2.26			$ 6.35	$ 6.08
Diluted earnings per common share (in dollars per share)		$ 7.17			$ 2.25			$ 6.31	$ 6.04
Net Cash Provided by (Used in) Investing Activities								$ (515,200,000)	$ (1,131,000,000.0)
Net Cash Provided by (Used in) Financing Activities								(516,400,000)	196,700,000
Effect of exchange rate changes on cash and cash equivalents								600,000	(6,300,000)
Assets		$ 18,739,600,000						18,739,600,000		18,046,400,000
Long-term debt, less current portion		3,009,900,000						3,009,900,000		2,655,800,000
Common stock, 92.8 and 93.5 shares outstanding at March 31, 2013 and December 31, 2012, respectively		19,900,000						19,900,000		20,100,000
Stockholders' Equity Attributable to Parent		8,239,100,000	7,352,700,000	7,020,100,000	$ 7,231,100,000	7,144,400,000	7,111,300,000	8,239,100,000	7,231,100,000	7,567,000,000.0	$ 6,971,400,000
Liabilities and Equity		18,739,600,000						18,739,600,000		18,046,400,000
Capitalized Contract Cost, Amortization		2,300,000			2,300,000			7,600,000	6,100,000
Deferred Revenue, Revenue Recognized								237,200,000	232,800,000
Revenue, Remaining Performance Obligation, Amount		4,795,000,000.0						4,795,000,000.0
Contract with Customer, Performance Obligation Satisfied in Previous Period								44,800,000	67,700,000
Goodwill, Impairment Loss								422,400,000
Other than Temporary Impairment Losses, Investments								25,400,000
Cash Payment Received Appropriated by U.S. Congress Covid-19	$ 55,900,000	76,200,000
CARES Act Funds Accrued Liabilities And Other Liabilities		132,100,000						132,100,000
COVID-19 Pandemic Goodwill and Intangible Asset Impairment								$ 437,400,000
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		$ 2,704,200,000			$ 1,759,200,000			$ 6,098,800,000	$ 5,242,100,000
Percent of Revenue Contributed		68.00%			60.00%			63.00%	61.00%
Goodwill, Impairment Loss								$ 3,700,000
Impairment of Intangible Assets (Excluding Goodwill)								30,500,000
Covance Drug Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues		$ 1,241,900,000			$ 1,175,400,000			$ 3,479,400,000	$ 3,376,400,000
Percent of Revenue Contributed		32.00%						37.00%
Goodwill, Impairment Loss								$ 418,700,000
Equity Method Income (Loss), Net [Domain]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other than Temporary Impairment Losses, Investments				7,100,000
Other Nonoperating Income (Expense) [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other than Temporary Impairment Losses, Investments			5,200,000	13,100,000
Retained Earnings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net Income (Loss) Attributable to Parent		$ 703,400,000	231,600,000	(317,200,000)		190,400,000	185,600,000
Stockholders' Equity Attributable to Parent		$ 8,464,000,000.0	$ 7,760,600,000	$ 7,529,000,000.0	$ 7,676,500,000	$ 7,455,800,000	$ 7,265,400,000	$ 8,464,000,000.0	$ 7,676,500,000	$ 7,903,600,000	$ 7,079,800,000