PENSION AND OTHER POST-EMPLOYMENT BENEFIT PLANS - Reconciliation of Pension Plan Assets Measured at Level 3 Fair Value (Details) - Fair Value, Inputs, Level 3 - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year (estimated)	$ 99	$ 330
Realized Gain (Loss)	3	237
Unrealized Gain (Loss)	(5)	(263)
Purchases, Sales, Settlements, Net	(18)	(243)
Transfers In (Out), Net	(7)	38
Fair value of plan assets at end of year (estimated)	72	99
Hedge funds and related investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year (estimated)	13	3
Realized Gain (Loss)	3	1
Unrealized Gain (Loss)	(4)	(1)
Purchases, Sales, Settlements, Net	(9)	(3)
Transfers In (Out), Net	1	13
Fair value of plan assets at end of year (estimated)	4	13
Private equity and related investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year (estimated)	86	65
Realized Gain (Loss)	0	(1)
Unrealized Gain (Loss)	(1)	0
Purchases, Sales, Settlements, Net	(9)	(3)
Transfers In (Out), Net	(8)	25
Fair value of plan assets at end of year (estimated)	68	86
Derivative instruments, net
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year (estimated)	$ 0	262
Realized Gain (Loss)		237
Unrealized Gain (Loss)		(262)
Purchases, Sales, Settlements, Net		(237)
Transfers In (Out), Net		0
Fair value of plan assets at end of year (estimated)		$ 0

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

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Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

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- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

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Amount, measured using unobservable input, of increase (decrease) in plan asset of defined benefit plan from purchase, sale and settlement of trade associated with underlying investment.

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Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan, for assets transferred into (out of) Level 3 of fair value hierarchy. Includes, but is not limited to, transfer due to change in observability of significant inputs.

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