RETIREMENT BENEFITS - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		$ 200,200
Change, net of tax	[1]	714	$ (286)
Balance, end of period		203,273	193,326	$ 200,200
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(6,864)	(6,809)	(6,809)
Actuarial assumptions changes and plan experience		1,430		(1,464)
Net asset (loss) gain due to difference between actual and expected returns		(718)		1,076
Net amortization		81		318
Prior service credit		0		108
Curtailment/settlement gain		0		(8)
Foreign exchange impact and other		114		(108)
Change in deferred taxes, net		(193)		23
Change, net of tax		714		(55)
Balance, end of period		$ (6,150)	$ (7,095)	$ (6,864)

[1] See Note 17 to the Consolidated Financial Statements.