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REVENUE RECOGNITION (Details) - USD ($) $ in Millions	12 Months Ended
REVENUE RECOGNITION (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Percentage of net revenues	100.00%	100.00%	100.00%
Percentage of net accounts receivable	100.00%	100.00%
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 15
Provision for credit losses	19
Write-offs of accounts receivable, net of recoveries	(6)
Balance, December 31, 2020	$ 28	$ 15
Healthcare insurers: \| Minimum
Segment Reporting Information [Line Items]
Collection of consideration	30 days
Healthcare insurers: \| Maximum
Segment Reporting Information [Line Items]
Collection of consideration	60 days
Government payers
Segment Reporting Information [Line Items]
Collection of consideration	30 days
Client payers \| Minimum
Segment Reporting Information [Line Items]
Collection of consideration	60 days
Client payers \| Maximum
Segment Reporting Information [Line Items]
Collection of consideration	90 days
Patients \| Minimum
Segment Reporting Information [Line Items]
Collection of consideration	30 days
Patients \| Maximum
Segment Reporting Information [Line Items]
Collection of consideration	60 days
DS \| Minimum
Segment Reporting Information [Line Items]
Collection of consideration	30 days
DS \| Maximum
Segment Reporting Information [Line Items]
Collection of consideration	60 days
DIS business
Segment Reporting Information [Line Items]
Percentage of net revenues	97.00%	96.00%	96.00%
Percentage of net accounts receivable	97.00%	95.00%
DIS business \| Minimum
Segment Reporting Information [Line Items]
Percentage of net revenues	95.00%	95.00%	95.00%
DIS business \| Healthcare insurers:
Segment Reporting Information [Line Items]
Percentage of net revenues	37.00%	36.00%	35.00%
Percentage of net accounts receivable	34.00%	22.00%
DIS business \| Healthcare insurers: \| Fee-for-service
Segment Reporting Information [Line Items]
Percentage of net revenues	34.00%	33.00%	32.00%
DIS business \| Healthcare insurers: \| Capitated
Segment Reporting Information [Line Items]
Percentage of net revenues	3.00%	3.00%	3.00%
DIS business \| Government payers
Segment Reporting Information [Line Items]
Percentage of net revenues	11.00%	15.00%	16.00%
Percentage of net accounts receivable	6.00%	11.00%
DIS business \| Client payers
Segment Reporting Information [Line Items]
Percentage of net revenues	38.00%	32.00%	32.00%
Percentage of net accounts receivable	46.00%	42.00%
DIS business \| Patients
Segment Reporting Information [Line Items]
Percentage of net revenues	11.00%	13.00%	13.00%
Period of billing fully reserve	210 days
Percentage of net accounts receivable	11.00%	20.00%
All other operating segments \| DS
Segment Reporting Information [Line Items]
Percentage of net revenues	3.00%	4.00%	4.00%
Percentage of net accounts receivable	3.00%	5.00%