RETIREMENT BENEFITS - Funded Status and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
U.S. | Pension Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	$ 13,815
Benefits earned during the period	0	$ 0
Interest cost on benefit obligation	82	106
Change in plan assets
Plan assets at fair value at beginning of year	13,309
Net amount recognized at period end
Benefit asset	730
Benefit liability	(663)
Net amount recognized on the balance sheet—Significant Plans	67
Amounts recognized in AOCI at period end
Prior service benefit	0
Net actuarial (loss) gain	(6,627)
Net amount recognized in equity-pretax	(6,627)
Accumulated benefit obligation at period end—Significant Plans	12,804
U.S. | Pension Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	25
Change in plan assets
Plan assets at fair value at beginning of year	0
U.S. | Pension Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	13,790
Benefits earned during the period	0
Interest cost on benefit obligation	82
Actuarial gain	(849)
Benefits paid, net of participants’ contributions and government subsidy	(216)
Foreign exchange impact and other	0
Projected benefit obligation at period end—Significant Plans	12,807
Change in plan assets
Plan assets at fair value at beginning of year	13,309
Actual return on plan assets	(232)
Company contributions, net of reimbursements	13
Benefits paid, net of participants’ contributions and government subsidy	(216)
Foreign exchange impact and other	0
Plan assets at fair value at period end—Significant Plans	12,874
Funded status of the Significant Plans	67
U.S. | Postretirement Benefit Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	559
Benefits earned during the period	0	0
Interest cost on benefit obligation	3	5
Change in plan assets
Plan assets at fair value at beginning of year	331
Net amount recognized at period end
Benefit asset	0
Benefit liability	(199)
Net amount recognized on the balance sheet—Significant Plans	(199)
Amounts recognized in AOCI at period end
Prior service benefit	99
Net actuarial (loss) gain	78
Net amount recognized in equity-pretax	177
Accumulated benefit obligation at period end—Significant Plans	522
U.S. | Postretirement Benefit Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	0
Change in plan assets
Plan assets at fair value at beginning of year	0
U.S. | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	559
Benefits earned during the period	0
Interest cost on benefit obligation	3
Actuarial gain	(31)
Benefits paid, net of participants’ contributions and government subsidy	(9)
Foreign exchange impact and other	0
Projected benefit obligation at period end—Significant Plans	522
Change in plan assets
Plan assets at fair value at beginning of year	331
Actual return on plan assets	(4)
Company contributions, net of reimbursements	5
Benefits paid, net of participants’ contributions and government subsidy	(9)
Foreign exchange impact and other	0
Plan assets at fair value at period end—Significant Plans	323
Funded status of the Significant Plans	(199)
Non-U.S. plans | Pension Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	8,629
Benefits earned during the period	39	37
Interest cost on benefit obligation	62	64
Change in plan assets
Plan assets at fair value at beginning of year	7,831
Net amount recognized at period end
Benefit asset	705
Benefit liability	(587)
Net amount recognized on the balance sheet—Significant Plans	118
Amounts recognized in AOCI at period end
Prior service benefit	1
Net actuarial (loss) gain	(1,043)
Net amount recognized in equity-pretax	(1,042)
Accumulated benefit obligation at period end—Significant Plans	5,211
Non-U.S. plans | Pension Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	2,248
Change in plan assets
Plan assets at fair value at beginning of year	1,500
Non-U.S. plans | Pension Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	6,381
Benefits earned during the period	23
Interest cost on benefit obligation	52
Actuarial gain	(428)
Benefits paid, net of participants’ contributions and government subsidy	(84)
Foreign exchange impact and other	(135)
Projected benefit obligation at period end—Significant Plans	5,809
Change in plan assets
Plan assets at fair value at beginning of year	6,331
Actual return on plan assets	(230)
Company contributions, net of reimbursements	18
Benefits paid, net of participants’ contributions and government subsidy	(84)
Foreign exchange impact and other	(108)
Plan assets at fair value at period end—Significant Plans	5,927
Funded status of the Significant Plans	118
Non-U.S. plans | Postretirement Benefit Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,390
Benefits earned during the period	2	2
Interest cost on benefit obligation	25	$ 24
Change in plan assets
Plan assets at fair value at beginning of year	1,146
Net amount recognized at period end
Benefit asset	127
Benefit liability	0
Net amount recognized on the balance sheet—Significant Plans	127
Amounts recognized in AOCI at period end
Prior service benefit	55
Net actuarial (loss) gain	(221)
Net amount recognized in equity-pretax	(166)
Accumulated benefit obligation at period end—Significant Plans	967
Non-U.S. plans | Postretirement Benefit Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	277
Change in plan assets
Plan assets at fair value at beginning of year	8
Non-U.S. plans | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,113
Benefits earned during the period	1
Interest cost on benefit obligation	22
Actuarial gain	(123)
Benefits paid, net of participants’ contributions and government subsidy	(18)
Foreign exchange impact and other	(28)
Projected benefit obligation at period end—Significant Plans	967
Change in plan assets
Plan assets at fair value at beginning of year	1,138
Actual return on plan assets	4
Company contributions, net of reimbursements	0
Benefits paid, net of participants’ contributions and government subsidy	(18)
Foreign exchange impact and other	(30)
Plan assets at fair value at period end—Significant Plans	1,094
Funded status of the Significant Plans	127
Qualified plans | U.S. | Pension Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	730
Qualified plans | U.S. | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	(199)
Qualified plans | Non-U.S. plans | Pension Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	118
Qualified plans | Non-U.S. plans | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	127
Nonqualified plans | U.S. | Pension Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	(663)
Nonqualified plans | U.S. | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	0
Nonqualified plans | Non-U.S. plans | Pension Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	0
Nonqualified plans | Non-U.S. plans | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	$ 0