RETIREMENT BENEFITS - Fair Value Disclosure (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Defined Benefit Plan Disclosure
Plan assets valued at NAV
U.S.
Defined Benefit Plan Disclosure
Other investment liabilities	$ (354)	$ (403)
Total net assets	11,835	12,987
U.S. | Level 1
Defined Benefit Plan Disclosure
Other investment liabilities	(100)	(60)
U.S. | Level 2
Defined Benefit Plan Disclosure
Other investment liabilities	(254)	(343)
U.S. | Level 3
Defined Benefit Plan Disclosure
Other investment liabilities	0	0
U.S. | U.S. equity
Defined Benefit Plan Disclosure
Total assets	625	726
U.S. | U.S. equity | Level 1
Defined Benefit Plan Disclosure
Total assets	625	726
U.S. | U.S. equity | Level 2
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | U.S. equity | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Non-U.S. equities
Defined Benefit Plan Disclosure
Total assets	481	821
U.S. | Non-U.S. equities | Level 1
Defined Benefit Plan Disclosure
Total assets	481	821
U.S. | Non-U.S. equities | Level 2
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Non-U.S. equities | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Mutual funds and other registered investment companies
Defined Benefit Plan Disclosure
Total assets	215	376
U.S. | Mutual funds and other registered investment companies | Level 1
Defined Benefit Plan Disclosure
Total assets	215	376
U.S. | Mutual funds and other registered investment companies | Level 2
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Mutual funds and other registered investment companies | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Commingled funds
Defined Benefit Plan Disclosure
Total assets	1,344	1,184
U.S. | Commingled funds | Level 1
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Commingled funds | Level 2
Defined Benefit Plan Disclosure
Total assets	1,344	1,184
U.S. | Commingled funds | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Debt securities
Defined Benefit Plan Disclosure
Total assets	4,821	4,461
U.S. | Debt securities | Level 1
Defined Benefit Plan Disclosure
Total assets	1,346	1,381
U.S. | Debt securities | Level 2
Defined Benefit Plan Disclosure
Total assets	3,475	3,080
U.S. | Debt securities | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Annuity contracts
Defined Benefit Plan Disclosure
Total assets	1	1
U.S. | Annuity contracts | Level 1
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Annuity contracts | Level 2
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Annuity contracts | Level 3
Defined Benefit Plan Disclosure
Total assets	1	1	$ 1
U.S. | Derivatives
Defined Benefit Plan Disclosure
Total assets	268	334
U.S. | Derivatives | Level 1
Defined Benefit Plan Disclosure
Total assets	16	11
U.S. | Derivatives | Level 2
Defined Benefit Plan Disclosure
Total assets	252	323
U.S. | Derivatives | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Other investments
Defined Benefit Plan Disclosure
Total assets	127	148
U.S. | Other investments | Level 1
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Other investments | Level 2
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Other investments | Level 3
Defined Benefit Plan Disclosure
Total assets	127	148	129
U.S. | Total investments
Defined Benefit Plan Disclosure
Total assets	7,882	8,051
Net investments at fair value	8,486	8,909
U.S. | Total investments | Level 1
Defined Benefit Plan Disclosure
Total assets	2,683	3,315
Net investments at fair value	2,676	3,512
U.S. | Total investments | Level 2
Defined Benefit Plan Disclosure
Total assets	5,071	4,587
Net investments at fair value	5,682	5,248
U.S. | Total investments | Level 3
Defined Benefit Plan Disclosure
Total assets	128	149	130
Net investments at fair value	128	149
U.S. | Cash and short-term investments
Defined Benefit Plan Disclosure
Total assets	958	1,261
U.S. | Cash and short-term investments | Level 1
Defined Benefit Plan Disclosure
Total assets	93	257
U.S. | Cash and short-term investments | Level 2
Defined Benefit Plan Disclosure
Total assets	865	1,004
U.S. | Cash and short-term investments | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
U.S. | Other investment receivables
Defined Benefit Plan Disclosure
Plan assets valued at NAV	80	16
U.S. | Securities
Defined Benefit Plan Disclosure
Plan assets valued at NAV	3,269	4,062
Non-U.S.
Defined Benefit Plan Disclosure
Other investment liabilities	(2,590)	(4,222)
Total net assets	7,735	8,247
Non-U.S. | Level 1
Defined Benefit Plan Disclosure
Other investment liabilities	(1)	(2)
Non-U.S. | Level 2
Defined Benefit Plan Disclosure
Other investment liabilities	(2,589)	(4,220)
Non-U.S. | Level 3
Defined Benefit Plan Disclosure
Other investment liabilities	0	0
Non-U.S. | U.S. equity
Defined Benefit Plan Disclosure
Total assets	13	16
Non-U.S. | U.S. equity | Level 1
Defined Benefit Plan Disclosure
Total assets	4	4
Non-U.S. | U.S. equity | Level 2
Defined Benefit Plan Disclosure
Total assets	9	12
Non-U.S. | U.S. equity | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Non-U.S. equities
Defined Benefit Plan Disclosure
Total assets	200	226
Non-U.S. | Non-U.S. equities | Level 1
Defined Benefit Plan Disclosure
Total assets	100	103
Non-U.S. | Non-U.S. equities | Level 2
Defined Benefit Plan Disclosure
Total assets	100	122
Non-U.S. | Non-U.S. equities | Level 3
Defined Benefit Plan Disclosure
Total assets	0	1	1
Non-U.S. | Mutual funds and other registered investment companies
Defined Benefit Plan Disclosure
Total assets	2,950	3,172
Non-U.S. | Mutual funds and other registered investment companies | Level 1
Defined Benefit Plan Disclosure
Total assets	2,887	3,098
Non-U.S. | Mutual funds and other registered investment companies | Level 2
Defined Benefit Plan Disclosure
Total assets	63	74
Non-U.S. | Mutual funds and other registered investment companies | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Commingled funds
Defined Benefit Plan Disclosure
Total assets	21	24
Non-U.S. | Commingled funds | Level 1
Defined Benefit Plan Disclosure
Total assets	21	24
Non-U.S. | Commingled funds | Level 2
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Commingled funds | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Debt securities
Defined Benefit Plan Disclosure
Total assets	6,654	5,561
Non-U.S. | Debt securities | Level 1
Defined Benefit Plan Disclosure
Total assets	5,145	3,999
Non-U.S. | Debt securities | Level 2
Defined Benefit Plan Disclosure
Total assets	1,500	1,555
Non-U.S. | Debt securities | Level 3
Defined Benefit Plan Disclosure
Total assets	9	7	7
Non-U.S. | Real estate
Defined Benefit Plan Disclosure
Total assets	4	4
Non-U.S. | Real estate | Level 1
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Real estate | Level 2
Defined Benefit Plan Disclosure
Total assets	3	3
Non-U.S. | Real estate | Level 3
Defined Benefit Plan Disclosure
Total assets	1	1	1
Non-U.S. | Annuity contracts
Defined Benefit Plan Disclosure
Total assets	11	10
Non-U.S. | Annuity contracts | Level 1
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Annuity contracts | Level 2
Defined Benefit Plan Disclosure
Total assets	1	1
Non-U.S. | Annuity contracts | Level 3
Defined Benefit Plan Disclosure
Total assets	10	9	8
Non-U.S. | Derivatives
Defined Benefit Plan Disclosure
Total assets	156	3,103
Non-U.S. | Derivatives | Level 1
Defined Benefit Plan Disclosure
Total assets	0	1
Non-U.S. | Derivatives | Level 2
Defined Benefit Plan Disclosure
Total assets	156	3,102
Non-U.S. | Derivatives | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Other investments
Defined Benefit Plan Disclosure
Total assets	211	215
Non-U.S. | Other investments | Level 1
Defined Benefit Plan Disclosure
Total assets	1	1
Non-U.S. | Other investments | Level 2
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Other investments | Level 3
Defined Benefit Plan Disclosure
Total assets	210	214	187
Non-U.S. | Total investments
Defined Benefit Plan Disclosure
Total assets	10,220	12,331
Net investments at fair value	7,724	8,231
Non-U.S. | Total investments | Level 1
Defined Benefit Plan Disclosure
Total assets	8,158	7,230
Net investments at fair value	8,248	7,347
Non-U.S. | Total investments | Level 2
Defined Benefit Plan Disclosure
Total assets	1,832	4,869
Net investments at fair value	(754)	652
Non-U.S. | Total investments | Level 3
Defined Benefit Plan Disclosure
Total assets	230	232	204
Net investments at fair value	230	232
Non-U.S. | Cash and short-term investments
Defined Benefit Plan Disclosure
Total assets	94	122
Non-U.S. | Cash and short-term investments | Level 1
Defined Benefit Plan Disclosure
Total assets	91	119
Non-U.S. | Cash and short-term investments | Level 2
Defined Benefit Plan Disclosure
Total assets	3	3
Non-U.S. | Cash and short-term investments | Level 3
Defined Benefit Plan Disclosure
Total assets	0	0
Non-U.S. | Securities
Defined Benefit Plan Disclosure
Plan assets valued at NAV	11	16
Pension Plans | U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 11,490	$ 12,725	12,363
Allocable interest (as a percent)	98.00%	99.00%
Pension Plans | Non-U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 6,699	$ 7,128	6,149
Postretirement Benefit Plans | U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 345	$ 262	129
Allocable interest (as a percent)	2.00%	1.00%
Postretirement Benefit Plans | Non-U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 1,036	$ 1,119	$ 1,015