POSTRETIREMENT BENEFIT PLANS - Components of Expense (Income) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Non-operating		$ (382)	$ (5)	$ 3
Pension Plans | Principal Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		32
Interest cost		952
Expected return on plan assets		(1,170)
Amortization of net loss (gain)		(125)
Amortization of prior service cost (credit)		4
Curtailment loss (gain)		17
Settlement loss (gain)		0
Non-operating		(322)
Net periodic expense (income)		(290)
Pension Plans | Other Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		23	19	24
Interest cost		209	17	15
Expected return on plan assets		(256)	(27)	(27)
Amortization of net loss (gain)		10	5	17
Amortization of prior service cost (credit)		(3)	(5)	(4)
Curtailment loss (gain)		0	0	0
Settlement loss (gain)	$ 61	61	0	0
Non-operating		21	(10)	1
Net periodic expense (income)		44	$ 9	$ 25
OPEB Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		6
Interest cost		59
Expected return on plan assets		0
Amortization of net loss (gain)		(64)
Amortization of prior service cost (credit)		(87)
Curtailment loss (gain)		0
Settlement loss (gain)		0
Non-operating		(92)
Net periodic expense (income)		$ (86)