RETIREMENT BENEFITS - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Change in accumulated other comprehensive income (loss)
Balance, beginning of period				$ 200,200
Change, net of tax	[1]	$ 135	$ 246	936	$ (117)
Balance, end of period		201,610	194,595	201,610	194,595
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(6,063)	(7,172)	(6,864)	(6,809)
Actuarial assumptions changes and plan experience		175		1,125
Net asset loss due to difference between actual and expected returns		(116)		(325)
Net amortization		69		216
Curtailment/settlement gain		1		(3)
Foreign exchange impact and other		46		153
Change in deferred taxes, net		(40)		(230)
Change, net of tax		135		936
Balance, end of period		$ (5,928)	$ (6,926)	$ (5,928)	$ (6,926)

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