RETIREMENT BENEFITS - Funded Status and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
U.S. | Pension Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	$ 13,453
Benefits earned during the period	0	$ 0
Interest cost on benefit obligation	106	130
Change in plan assets
Plan assets at fair value at beginning of year	12,717
Net amount recognized at period end
Benefit asset	0
Benefit liability	(1,496)
Net amount recognized on the balance sheet—Significant Plans	(1,496)
Amounts recognized in AOCI at period end
Prior service benefit	0
Net actuarial (loss) gain	(7,932)
Net amount recognized in equity-pretax	(7,932)
Accumulated benefit obligation at period end—Significant Plans	13,344
U.S. | Pension Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	26
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	13,427
Benefits earned during the period
Interest cost on benefit obligation	106
Actuarial loss (gain)	65
Benefits paid, net of participants’ contributions and government subsidy	(249)
Foreign exchange impact and other	0
Projected benefit obligation at period end—Significant Plans	13,349
Change in plan assets
Plan assets at fair value at beginning of year	12,717
Actual return on plan assets	(628)
Company contributions, net of reimbursements	13
Benefits paid, net of participants’ contributions and government subsidy	(249)
Foreign exchange impact and other	0
Plan assets at fair value at period end—Significant Plans	11,853
Funded status of the Significant Plans	(1,496)
U.S. | Postretirement Benefit Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	692
Benefits earned during the period	0	0
Interest cost on benefit obligation	5	7
Change in plan assets
Plan assets at fair value at beginning of year	345
Net amount recognized at period end
Benefit asset	0
Benefit liability	(358)
Net amount recognized on the balance sheet—Significant Plans	(358)
Amounts recognized in AOCI at period end
Prior service benefit	0
Net actuarial (loss) gain	22
Net amount recognized in equity-pretax	22
Accumulated benefit obligation at period end—Significant Plans	679
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	692
Benefits earned during the period	0
Interest cost on benefit obligation	5
Actuarial loss (gain)	(13)
Benefits paid, net of participants’ contributions and government subsidy	(5)
Foreign exchange impact and other	0
Projected benefit obligation at period end—Significant Plans	679
Change in plan assets
Plan assets at fair value at beginning of year	345
Actual return on plan assets	(11)
Company contributions, net of reimbursements	(8)
Benefits paid, net of participants’ contributions and government subsidy	(5)
Foreign exchange impact and other	0
Plan assets at fair value at period end—Significant Plans	321
Funded status of the Significant Plans	(358)
Non-U.S. plans | Pension Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	8,105
Benefits earned during the period	37	36
Interest cost on benefit obligation	64	75
Change in plan assets
Plan assets at fair value at beginning of year	7,556
Net amount recognized at period end
Benefit asset	1,001
Benefit liability	(617)
Net amount recognized on the balance sheet—Significant Plans	384
Amounts recognized in AOCI at period end
Prior service benefit	8
Net actuarial (loss) gain	(780)
Net amount recognized in equity-pretax	(772)
Accumulated benefit obligation at period end—Significant Plans	4,827
Non-U.S. plans | Pension Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	2,068
Change in plan assets
Plan assets at fair value at beginning of year	1,349
Non-U.S. plans | Pension Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	6,037
Benefits earned during the period	21
Interest cost on benefit obligation	55
Actuarial loss (gain)	(419)
Benefits paid, net of participants’ contributions and government subsidy	(69)
Foreign exchange impact and other	(522)
Projected benefit obligation at period end—Significant Plans	5,103
Change in plan assets
Plan assets at fair value at beginning of year	6,207
Actual return on plan assets	(156)
Company contributions, net of reimbursements	16
Benefits paid, net of participants’ contributions and government subsidy	(69)
Foreign exchange impact and other	(511)
Plan assets at fair value at period end—Significant Plans	5,487
Funded status of the Significant Plans	384
Non-U.S. plans | Postretirement Benefit Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,384
Benefits earned during the period	2	2
Interest cost on benefit obligation	24	$ 26
Change in plan assets
Plan assets at fair value at beginning of year	1,127
Net amount recognized at period end
Benefit asset	42
Benefit liability	0
Net amount recognized on the balance sheet—Significant Plans	42
Amounts recognized in AOCI at period end
Prior service benefit	54
Net actuarial (loss) gain	(289)
Net amount recognized in equity-pretax	(235)
Accumulated benefit obligation at period end—Significant Plans	806
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	323
Change in plan assets
Plan assets at fair value at beginning of year	9
Non-U.S. plans | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,061
Benefits earned during the period	1
Interest cost on benefit obligation	21
Actuarial loss (gain)	(63)
Benefits paid, net of participants’ contributions and government subsidy	(12)
Foreign exchange impact and other	(202)
Projected benefit obligation at period end—Significant Plans	806
Change in plan assets
Plan assets at fair value at beginning of year	1,118
Actual return on plan assets	(45)
Company contributions, net of reimbursements	0
Benefits paid, net of participants’ contributions and government subsidy	(12)
Foreign exchange impact and other	(213)
Plan assets at fair value at period end—Significant Plans	848
Funded status of the Significant Plans	42
Qualified plans | U.S. | Pension Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	(818)
Qualified plans | U.S. | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	(358)
Qualified plans | Non-U.S. plans | Pension Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	384
Qualified plans | Non-U.S. plans | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	42
Nonqualified plans | U.S. | Pension Plans | Significant Plans Measured Quarterly
Change in plan assets
Funded status of the Significant Plans	(678)
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