RETIREMENT BENEFITS - Funded Status and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
U.S. | Pension Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	$ 14,040
Benefits earned during the period	1	$ 1
Interest cost on benefit obligation	123	139
Projected benefit obligation at period end—Significant Plans	13,436
Change in plan assets
Plan assets at fair value at beginning of year	12,725
Plan assets at fair value at period end—Significant Plans	12,376
Funded status of the Significant Plans	(1,060)
Net amount recognized at period end
Benefit asset	0
Benefit liability	(1,060)
Net amount recognized on the balance sheet—Significant Plans	(1,060)
Amounts recognized in AOCI at period end
Prior service benefit	0
Net actuarial (loss) gain	(6,644)
Net amount recognized in equity-pretax	(6,644)
Accumulated benefit obligation at period end—Significant Plans	13,430
U.S. | Pension Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	28
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	14,012
Benefits earned during the period	1
Interest cost on benefit obligation	123
Actuarial (gain) loss	(495)
Benefits paid, net of participants’ contributions and government subsidy	(205)
Foreign exchange impact and other	0
Change in plan assets
Plan assets at fair value at beginning of year	12,725
Actual return on plan assets	(157)
Company contributions, net of reimbursements	13
Benefits paid, net of participants’ contributions and government subsidy	(205)
Foreign exchange impact and other	0
U.S. | Postretirement Benefit Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	699
Benefits earned during the period	0	0
Interest cost on benefit obligation	6	6
Projected benefit obligation at period end—Significant Plans	667
Change in plan assets
Plan assets at fair value at beginning of year	262
Plan assets at fair value at period end—Significant Plans	241
Funded status of the Significant Plans	(426)
Net amount recognized at period end
Benefit asset	0
Benefit liability	(426)
Net amount recognized on the balance sheet—Significant Plans	(426)
Amounts recognized in AOCI at period end
Prior service benefit	0
Net actuarial (loss) gain	89
Net amount recognized in equity-pretax	89
Accumulated benefit obligation at period end—Significant Plans	667
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	699
Benefits earned during the period	0
Interest cost on benefit obligation	6
Actuarial (gain) loss	(21)
Benefits paid, net of participants’ contributions and government subsidy	(17)
Foreign exchange impact and other	0
Change in plan assets
Plan assets at fair value at beginning of year	262
Actual return on plan assets	0
Company contributions, net of reimbursements	(4)
Benefits paid, net of participants’ contributions and government subsidy	(17)
Foreign exchange impact and other	0
Non-U.S. plans | Pension Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	7,433
Benefits earned during the period	38	36
Interest cost on benefit obligation	75	71
Projected benefit obligation at period end—Significant Plans	5,597
Change in plan assets
Plan assets at fair value at beginning of year	7,128
Plan assets at fair value at period end—Significant Plans	5,938
Funded status of the Significant Plans	341
Net amount recognized at period end
Benefit asset	829
Benefit liability	(488)
Net amount recognized on the balance sheet—Significant Plans	341
Amounts recognized in AOCI at period end
Prior service benefit	27
Net actuarial (loss) gain	(882)
Net amount recognized in equity-pretax	(855)
Accumulated benefit obligation at period end—Significant Plans	5,329
Non-U.S. plans | Pension Plans | Other than Significant Plans Measured Annually
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,987
Change in plan assets
Plan assets at fair value at beginning of year	1,305
Non-U.S. plans | Pension Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	5,446
Benefits earned during the period	22
Interest cost on benefit obligation	63
Actuarial (gain) loss	(6)
Benefits paid, net of participants’ contributions and government subsidy	(68)
Foreign exchange impact and other	140
Change in plan assets
Plan assets at fair value at beginning of year	5,823
Actual return on plan assets	26
Company contributions, net of reimbursements	11
Benefits paid, net of participants’ contributions and government subsidy	(68)
Foreign exchange impact and other	146
Non-U.S. plans | Postretirement Benefit Plans
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,261
Benefits earned during the period	2	2
Interest cost on benefit obligation	26	$ 24
Projected benefit obligation at period end—Significant Plans	1,016
Change in plan assets
Plan assets at fair value at beginning of year	1,119
Plan assets at fair value at period end—Significant Plans	1,167
Funded status of the Significant Plans	151
Net amount recognized at period end
Benefit asset	151
Benefit liability	0
Net amount recognized on the balance sheet—Significant Plans	151
Amounts recognized in AOCI at period end
Prior service benefit	86
Net actuarial (loss) gain	(364)
Net amount recognized in equity-pretax	(278)
Accumulated benefit obligation at period end—Significant Plans	1,016
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	334
Change in plan assets
Plan assets at fair value at beginning of year	10
Non-U.S. plans | Postretirement Benefit Plans | Significant Plans Measured Quarterly
Change in projected benefit obligation
Projected benefit obligation at beginning of year	927
Benefits earned during the period	2
Interest cost on benefit obligation	23
Actuarial (gain) loss	5
Benefits paid, net of participants’ contributions and government subsidy	(12)
Foreign exchange impact and other	71
Change in plan assets
Plan assets at fair value at beginning of year	1,109
Actual return on plan assets	(14)
Company contributions, net of reimbursements	0
Benefits paid, net of participants’ contributions and government subsidy	(12)
Foreign exchange impact and other	84
Qualified plans | U.S. | Pension Plans
Change in plan assets
Funded status of the Significant Plans	(365)
Qualified plans | U.S. | Postretirement Benefit Plans
Change in plan assets
Funded status of the Significant Plans	(426)
Qualified plans | Non-U.S. plans | Pension Plans
Change in plan assets
Funded status of the Significant Plans	341
Qualified plans | Non-U.S. plans | Postretirement Benefit Plans
Change in plan assets
Funded status of the Significant Plans	151
Nonqualified plans | U.S. | Pension Plans
Change in plan assets
Funded status of the Significant Plans	(695)
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