NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounts Receivable (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Rollforward of the allowance for doubtful accounts
Balance at beginning of period	$ 8	$ 2	$ 3
Charged to expense	2	9	5
Net write-offs	(5)	(3)	(6)
Balance at end of period	$ 5	$ 8	$ 2
Customer One \| Customer Concentration Risk \| Accounts Receivable
Rollforward of the allowance for doubtful accounts
Concentration percentage	19.00%	24.00%
Customer Two \| Customer Concentration Risk \| Accounts Receivable
Rollforward of the allowance for doubtful accounts
Concentration percentage	17.00%	15.00%
Customer Three \| Customer Concentration Risk \| Accounts Receivable
Rollforward of the allowance for doubtful accounts
Concentration percentage	15.00%	11.00%

Amount of allowance for credit loss on accounts receivable, classified as current.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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Amount of direct write-downs of accounts receivable charged against the allowance.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Amount of expense (reversal of expense) for expected credit loss on accounts receivable.

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