RETIREMENT BENEFITS - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		$ 193,946	$ 197,074
Change in deferred taxes, net		23	119	$ 20
Change, net of tax	[1]	55	552	74
Balance, end of year		200,200	193,946	197,074
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(6,809)	(6,257)	(6,183)
Actuarial assumptions changes and plan experience		(1,464)	(2,300)	1,288
Net asset gain (loss) due to difference between actual and expected returns		1,076	1,427	(1,732)
Net amortization		318	274	214
Prior service credit (cost)		108	(7)	(7)
Curtailment/settlement gain		(8)	1	7
Foreign exchange impact and other		(108)	(66)	136
Change in deferred taxes, net		23	119	20
Change, net of tax		55	552	74
Balance, end of year		$ (6,864)	$ (6,809)	$ (6,257)

[1] See Note 8 to the Consolidated Financial Statements.