POSTRETIREMENT BENEFIT PLANS - Components of Expense (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Defined Benefit Plan Disclosure [Line Items]
Non-operating	$ (102)	$ (115)
Pension Plans | Principal Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost – Operating	7	8
Interest cost	227	240
Expected return on plan assets	(283)	(293)
Amortization of net loss (gain)	(19)	(31)
Amortization of prior service cost (credit)	2	0
Non-operating	(73)	(84)
Net periodic expense (income)	(66)	(76)
Pension Plans | Other Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost – Operating	6	6
Interest cost	50	52
Expected return on plan assets	(63)	(63)
Amortization of net loss (gain)	5	2
Amortization of prior service cost (credit)	0	(1)
Non-operating	(8)	(10)
Net periodic expense (income)	(2)	(4)
OPEB Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost – Operating	2	2
Interest cost	13	15
Expected return on plan assets	0	0
Amortization of net loss (gain)	(15)	(16)
Amortization of prior service cost (credit)	(22)	(22)
Non-operating	(24)	(23)
Net periodic expense (income)	$ (22)	$ (21)