RETIREMENT BENEFITS - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		$ 206,251
Change, net of tax	[1],[2]	77	$ (104)
Balance, end of period		207,398	209,019	$ 206,251
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(6,050)	(5,755)	(5,755)
Actuarial assumptions changes and plan experience		280	(269)	(547)
Net asset (loss) gain due to difference between actual and expected returns		(271)	183	263
Net amortization		64	43	175
Prior service benefit		0	0	2
Curtailment/settlement (gain)		0	(5)	(7)
Foreign exchange impact and other		(5)	(108)	(239)
Change in deferred taxes, net		9	52	58
Change, net of tax		77	(104)	(295)
Balance, end of period		$ (5,973)	$ (5,859)	$ (6,050)

[1] See Note 8.

[2] See Note 19.