RETIREMENT BENEFITS - Funded Status and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
U.S. | Pension Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	$ 12,655
Benefits earned during the period	0	$ 1
Interest cost on benefit obligation	130	123
Projected benefit obligation at period end—Significant Plans	13,038
Change in plan assets
Plan assets at fair value at beginning of year	11,490
Plan assets at fair value at period end—Significant Plans	11,977
Funded status of the Significant Plans	(1,061)
Net amount recognized at period end
Benefit asset	0
Benefit liability	(1,061)
Net amount recognized on the balance sheet—Significant Plans	(1,061)
Amounts recognized in AOCI at period end
Prior service benefit	0
Net actuarial (loss) gain	(6,848)
Net amount recognized in equity-pretax	(6,848)
Accumulated benefit obligation at period end—Significant Plans	13,029
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	12,630
Benefits earned during the period	0
Interest cost on benefit obligation	130
Actuarial loss	493
Benefits paid, net of participants’ contributions and government subsidy	(215)
Foreign exchange impact and other	0
Change in plan assets
Plan assets at fair value at beginning of year	11,490
Actual return on plan assets	688
Company contributions, net of reimbursements	14
Benefits paid, net of participants’ contributions and government subsidy	(215)
Foreign exchange impact and other	0
U.S. | Postretirement Benefit Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	662
Benefits earned during the period	0	0
Interest cost on benefit obligation	7	6
Projected benefit obligation at period end—Significant Plans	675
Change in plan assets
Plan assets at fair value at beginning of year	345
Plan assets at fair value at period end—Significant Plans	347
Funded status of the Significant Plans	(328)
Net amount recognized at period end
Benefit asset	0
Benefit liability	(328)
Net amount recognized on the balance sheet—Significant Plans	(328)
Amounts recognized in AOCI at period end
Prior service benefit	0
Net actuarial (loss) gain	50
Net amount recognized in equity-pretax	50
Accumulated benefit obligation at period end—Significant Plans	675
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	662
Benefits earned during the period	0
Interest cost on benefit obligation	7
Actuarial loss	13
Benefits paid, net of participants’ contributions and government subsidy	(7)
Foreign exchange impact and other	0
Change in plan assets
Plan assets at fair value at beginning of year	345
Actual return on plan assets	15
Company contributions, net of reimbursements	(6)
Benefits paid, net of participants’ contributions and government subsidy	(7)
Foreign exchange impact and other	0
Non-U.S. plans | Pension Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	7,149
Benefits earned during the period	36	38
Interest cost on benefit obligation	75	75
Projected benefit obligation at period end—Significant Plans	5,580
Change in plan assets
Plan assets at fair value at beginning of year	6,699
Plan assets at fair value at period end—Significant Plans	5,708
Funded status of the Significant Plans	128
Net amount recognized at period end
Benefit asset	766
Benefit liability	(638)
Net amount recognized on the balance sheet—Significant Plans	128
Amounts recognized in AOCI at period end
Prior service benefit	15
Net actuarial (loss) gain	(978)
Net amount recognized in equity-pretax	(963)
Accumulated benefit obligation at period end—Significant Plans	5,302
Non-U.S. plans | Pension Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,862
Change in plan assets
Plan assets at fair value at beginning of year	1,248
Non-U.S. plans | Pension Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	5,287
Benefits earned during the period	20
Interest cost on benefit obligation	63
Actuarial loss	252
Benefits paid, net of participants’ contributions and government subsidy	(55)
Foreign exchange impact and other	13
Change in plan assets
Plan assets at fair value at beginning of year	5,451
Actual return on plan assets	273
Company contributions, net of reimbursements	14
Benefits paid, net of participants’ contributions and government subsidy	(55)
Foreign exchange impact and other	25
Non-U.S. plans | Postretirement Benefit Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,159
Benefits earned during the period	2	2
Interest cost on benefit obligation	26	$ 26
Projected benefit obligation at period end—Significant Plans	914
Change in plan assets
Plan assets at fair value at beginning of year	1,036
Plan assets at fair value at period end—Significant Plans	1,059
Funded status of the Significant Plans	145
Net amount recognized at period end
Benefit asset	145
Benefit liability	0
Net amount recognized on the balance sheet—Significant Plans	145
Amounts recognized in AOCI at period end
Prior service benefit	73
Net actuarial (loss) gain	(314)
Net amount recognized in equity-pretax	(241)
Accumulated benefit obligation at period end—Significant Plans	914
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	307
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	852
Benefits earned during the period	1
Interest cost on benefit obligation	23
Actuarial loss	38
Benefits paid, net of participants’ contributions and government subsidy	(11)
Foreign exchange impact and other	11
Change in plan assets
Plan assets at fair value at beginning of year	1,027
Actual return on plan assets	29
Company contributions, net of reimbursements	0
Benefits paid, net of participants’ contributions and government subsidy	(11)
Foreign exchange impact and other	14
Qualified plans | U.S. | Pension Plans
Change in plan assets
Funded status of the Significant Plans	(391)
Qualified plans | U.S. | Postretirement Benefit Plans
Change in plan assets
Funded status of the Significant Plans	(328)
Qualified plans | Non-U.S. plans | Pension Plans
Change in plan assets
Funded status of the Significant Plans	128
Qualified plans | Non-U.S. plans | Postretirement Benefit Plans
Change in plan assets
Funded status of the Significant Plans	145
Nonqualified plans | U.S. | Pension Plans
Change in plan assets
Funded status of the Significant Plans	(670)
Nonqualified plans | U.S. | Postretirement Benefit Plans
Change in plan assets
Funded status of the Significant Plans	0
Nonqualified plans | Non-U.S. plans | Pension Plans
Change in plan assets
Funded status of the Significant Plans	0
Nonqualified plans | Non-U.S. plans | Postretirement Benefit Plans
Change in plan assets
Funded status of the Significant Plans	$ 0