BUSINESS ACQUISITIONS (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jul. 31, 2017	Jul. 27, 2016	Feb. 19, 2015	Jan. 30, 2015	Dec. 19, 2014	Nov. 01, 2013
Business Acquisition [Line Items]
Stock Issued During Period, Shares, Acquisitions										15.3
Total acquisition consideration (cash and non-cash)														$ 1,224.5	$ 249.1	$ 6,150.7
Long-term debt, less current portion	$ 6,344.6				$ 5,300.0					$ 6,344.6	$ 5,300.0
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability								$ 4.9	$ 1.7	3.0
Employee Severance Benefits Related Restructuring Reserve Accrual Adjustment										0.5
Facility Related Restructuring Reserve Accrual Adjustment								3.5		4.6			$ 1.1
Business Acquisition, Share Price															$ 2.40
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable intangible assets acquired														0.9
Goodwill, net	7,530.0				6,424.4					7,530.0	6,424.4	$ 6,202.1
Payments to Acquire Businesses, Net of Cash Acquired										1,882.6	548.6	3,736.0
Finite-lived Intangible Assets Acquired										1,053.0
Goodwill, Acquired During Period								398.3		$ 1,009.8
Line of Credit Facility, Maximum Borrowing Capacity																		$ 1,000.0
Purchase Price Allocation [Abstract]
Weighted-average useful lives of identifiable intangible assets										19 years 3 months
Cash payments to acquire laboratory-related assets										$ 312.9	278.9	255.8
Senior Notes, Noncurrent	2,900.0									2,900.0							$ 2,900.0		$ 700.0
Noncontrolling Interest Put [Abstract]
Net sales	2,701.5	$ 2,597.9	$ 2,498.4	$ 2,408.1	2,387.3	$ 2,372.7	$ 2,382.0	2,295.2		10,205.9	9,437.2	8,505.7
Proforma consolidated net revenue								188.4		10,576.3	9,937.4
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax										1,377.0	1,327.2
Business Acquisition, Pro Forma Net Income (Loss)										$ 1,258.3	$ 714.3
Proforma consolidated net income								11.6
Business Acquisition, Pro Forma Earnings Per Share, Basic										$ 12,290,000	$ 6,970,000
Unbilled Receivables, Current	138.2				97.1					$ 138.2	$ 97.1
Revenues								9,641.8		10,441.4	9,641.8	8,680.1
Operating Income (Loss)										1,364.2	1,312.4	996.8
Net earnings attributable to Laboratory Corporation of America Holdings	706.8	$ 180.6	$ 188.6	$ 192.2	184.4	179.5	$ 204.1	164.1		1,268.2	$ 732.1	437.6
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents														30.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables														105.6
Purchase price adjustment										$ 0.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets														57.9
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment														12.1
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net														(760.5)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable														45.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities														23.4
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue														124.2
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other														1.1
Business Combination, Separately Recognized Transactions, Liabilities Recognized														414.4
Proforma consolidated diluted earnings per share										$ 12,110,000	$ 6,850,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets														32.6
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets														1,638.9
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent														220.6
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net														1,224.5
Senior notes due 2025 [Member]
Purchase Price Allocation [Abstract]
Debt Instrument, Interest Rate, Stated Percentage																	3.60%
Senior Notes, Noncurrent																	$ 1,000.0
Senior notes due 2020 [Member]
Business Acquisition [Line Items]
Long-term debt, less current portion	600.0									$ 600.0
Covance, Inc. [Member]
Noncontrolling Interest Put [Abstract]
Proforma consolidated net revenue												9,033.3
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax												1,117.2
Business Acquisition, Pro Forma Net Income (Loss)												$ 547.5
Business Acquisition, Pro Forma Earnings Per Share, Basic												$ 5,050,000
Proforma consolidated diluted earnings per share												$ 5,030,000
Chiltern [Member]
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable intangible assets acquired														0.0
Noncontrolling Interest Put [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents														30.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables														116.9
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets														57.9
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment														12.1
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net														(676.6)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable														18.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities														51.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue														124.2
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other														1.1
Business Combination, Separately Recognized Transactions, Liabilities Recognized														402.4
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets														32.6
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets														1,626.9
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent														208.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net														1,224.5
Sequenom acquisition [Member]
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable deferred tax liabilities assumed					45.1						$ 45.1
Finite-lived Intangible Assets Acquired											146.6
Goodwill, Acquired During Period											$ 206.0
Purchase Price Allocation [Abstract]
Weighted-average useful lives of identifiable intangible assets											14 years 7 months
Excluding Covance [Member]
Cost of Acquired Entity [Abstract]
Payments to Acquire Businesses, Net of Cash Acquired										688.8	$ 299.5
Finite-lived Intangible Assets Acquired										1,053.0	126.2
Goodwill, Acquired During Period										1,009.8	192.3
Acquisitions excluding LipoScience [Member]
Cost of Acquired Entity [Abstract]
Payments to Acquire Businesses, Net of Cash Acquired												$ 128.4
Finite-lived Intangible Assets Acquired												17.4
Goodwill, Acquired During Period												68.4
Covance [Member]
Business Acquisition [Line Items]
Cash on Hand Used for Acquisition																$ 488.2
Business Acquisition, Share Price																$ 75.76
Noncash or Part Noncash Acquisition, Noncash Financial or Equity Instrument Consideration, Rate				26.86%
Acquisition Financing																$ 3,900,000.0
Business Acquisition, Transaction Costs	$ 25.4				6.9					25.4	6.9	$ 166.0
Purchase Price Allocation [Abstract]
Net Proceeds from Debt																	$ 2,870.2
Selling, General and Administrative Expenses [Member]
Business Acquisition [Line Items]
Other Special Charges								$ 25.6		1.3	9.0
Selling, General and Administrative Expenses [Member] | Covance [Member]
Business Acquisition [Line Items]
Business Acquisition, Transaction Costs					113.4						113.4
Interest Expense [Member] | Covance [Member]
Business Acquisition [Line Items]
Business Acquisition, Transaction Costs					$ 52.6						$ 52.6
60-Day Debt Bridge Traunche [Domain]
Cost of Acquired Entity [Abstract]
Bridge Term Credit Facility Agreement, Maximum Borrowing Amount																		$ 400.0
Short-term Debt, Terms								P60D
Cash and Cash Equivalents [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										0.0
Accrued Liabilities [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										(27.6)
Deferred Revenue [Domain]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										0.0
Deferred tax liability [Domain]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										12.6
Other Liabilities [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										0.0
Accounts Receivable [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										(11.3)
Prepaid Expenses and Other Current Assets [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										0.0
Property, Plant and Equipment [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										0.0
Goodwill [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										83.9
Assets [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										12.0
Accounts Payable [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										27.0
Liability [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										12.0
Other Current Assets [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										$ 0.0
Software and Software Development Costs [Member] | Chiltern [Member]
Purchase Price Allocation [Abstract]
Weighted-average useful lives of identifiable intangible assets										6 years
Use Rights [Member]
Cost of Acquired Entity [Abstract]
Finite-lived Intangible Assets Acquired										$ 0.9
Technology-Based Intangible Assets [Member]
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable intangible assets acquired														26.0
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										(21.0)
Technology-Based Intangible Assets [Member] | Chiltern [Member]
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable intangible assets acquired														47.0
Trademarks and Trade Names [Member]
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable intangible assets acquired														10.6
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										$ (13.5)
Trademarks and Trade Names [Member] | Chiltern [Member]
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable intangible assets acquired														24.1
Purchase Price Allocation [Abstract]
Weighted-average useful lives of identifiable intangible assets										4 years
Trade Names [Member]
Cost of Acquired Entity [Abstract]
Finite-lived Intangible Assets Acquired										$ 15.7
Purchase Price Allocation [Abstract]
Weighted-average useful lives of identifiable intangible assets										5 years 8 months
Customer Relationships [Member]
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable intangible assets acquired														602.0
Finite-lived Intangible Assets Acquired										$ 955.7
Acquired Finite-lived Intangible Assets, Useful Life										21 years
Purchase Price Allocation [Abstract]
Weighted-average useful lives of identifiable intangible assets										20 years 8 months
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										$ (27.0)
Customer Relationships [Member] | Chiltern [Member]
Cost of Acquired Entity [Abstract]
Business combination, recognized identifiable intangible assets acquired														$ 629.0
Off-Market Favorable Lease [Member]
Noncontrolling Interest Put [Abstract]
Purchase price adjustment										$ 0.9
Senior notes due 2018 [Member]
Business Acquisition [Line Items]
Long-term debt, less current portion						$ 130.0