RETIREMENT BENEFITS - Fair Value Disclosure (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 13,640	$ 13,062
U.S. | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	9,946	9,730
U.S. | Level 1
Defined Benefit Plan Disclosure
Total net assets	3,354	3,122
U.S. | Level 2
Defined Benefit Plan Disclosure
Total net assets	6,534	6,532
U.S. | Level 3
Defined Benefit Plan Disclosure
Total net assets	58	76	$ 128
U.S. | Total investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	8,851	8,655
U.S. | Total investments | Level 1
Defined Benefit Plan Disclosure
Total net assets	3,284	3,116
U.S. | Total investments | Level 2
Defined Benefit Plan Disclosure
Total net assets	5,509	5,463
U.S. | Total investments | Level 3
Defined Benefit Plan Disclosure
Total net assets	58	76
U.S. | U.S. equity | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	813	739
U.S. | U.S. equity | Level 1
Defined Benefit Plan Disclosure
Total net assets	813	739
U.S. | U.S. equity | Level 2
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | U.S. equity | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Non-U.S. equities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	725	553
U.S. | Non-U.S. equities | Level 1
Defined Benefit Plan Disclosure
Total net assets	725	553
U.S. | Non-U.S. equities | Level 2
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Non-U.S. equities | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Mutual funds and other registered investment companies | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	447	280
U.S. | Mutual funds and other registered investment companies | Level 1
Defined Benefit Plan Disclosure
Total net assets	447	280
U.S. | Mutual funds and other registered investment companies | Level 2
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Mutual funds and other registered investment companies | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Commingled funds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	1,074	1,410
U.S. | Commingled funds | Level 1
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Commingled funds | Level 2
Defined Benefit Plan Disclosure
Total net assets	1,074	1,410
U.S. | Commingled funds | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Debt securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	5,704	5,580
U.S. | Debt securities | Level 1
Defined Benefit Plan Disclosure
Total net assets	1,275	1,534
U.S. | Debt securities | Level 2
Defined Benefit Plan Disclosure
Total net assets	4,429	4,046
U.S. | Debt securities | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Annuity contracts | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	1	1
U.S. | Annuity contracts | Level 1
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Annuity contracts | Level 2
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Annuity contracts | Level 3
Defined Benefit Plan Disclosure
Total net assets	1	1	1
U.S. | Derivatives | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	14	17
U.S. | Derivatives | Level 1
Defined Benefit Plan Disclosure
Total net assets	8	10
U.S. | Derivatives | Level 2
Defined Benefit Plan Disclosure
Total net assets	6	7
U.S. | Derivatives | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Other investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	73	75
U.S. | Other investments | Level 1
Defined Benefit Plan Disclosure
Total net assets	16	0
U.S. | Other investments | Level 2
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Other investments | Level 3
Defined Benefit Plan Disclosure
Total net assets	57	75	127
U.S. | Cash and short-term investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	1,107	1,173
U.S. | Cash and short-term investments | Level 1
Defined Benefit Plan Disclosure
Total net assets	72	93
U.S. | Cash and short-term investments | Level 2
Defined Benefit Plan Disclosure
Total net assets	1,035	1,080
U.S. | Cash and short-term investments | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Other investment liabilities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	(12)	(98)
U.S. | Other investment liabilities | Level 1
Defined Benefit Plan Disclosure
Total net assets	(2)	(87)
U.S. | Other investment liabilities | Level 2
Defined Benefit Plan Disclosure
Total net assets	(10)	(11)
U.S. | Other investment liabilities | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
U.S. | Other investment receivables | Net Asset Value
Defined Benefit Plan Disclosure
Total net assets	99	22
U.S. | Securities | Net Asset Value
Defined Benefit Plan Disclosure
Total net assets	3,595	3,310
Non-U.S.
Defined Benefit Plan Disclosure
Total net assets	8,977	8,683
Non-U.S. | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	8,963	8,668
Non-U.S. | Level 1
Defined Benefit Plan Disclosure
Total net assets	10,105	7,768
Non-U.S. | Level 2
Defined Benefit Plan Disclosure
Total net assets	(1,461)	610
Non-U.S. | Level 3
Defined Benefit Plan Disclosure
Total net assets	319	290	230
Non-U.S. | Total investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	13,481	11,520
Non-U.S. | Total investments | Level 1
Defined Benefit Plan Disclosure
Total net assets	9,976	7,685
Non-U.S. | Total investments | Level 2
Defined Benefit Plan Disclosure
Total net assets	3,186	3,545
Non-U.S. | Total investments | Level 3
Defined Benefit Plan Disclosure
Total net assets	319	290
Non-U.S. | U.S. equity | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	21	16
Non-U.S. | U.S. equity | Level 1
Defined Benefit Plan Disclosure
Total net assets	5	4
Non-U.S. | U.S. equity | Level 2
Defined Benefit Plan Disclosure
Total net assets	16	12
Non-U.S. | U.S. equity | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Non-U.S. equities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	775	389
Non-U.S. | Non-U.S. equities | Level 1
Defined Benefit Plan Disclosure
Total net assets	105	127
Non-U.S. | Non-U.S. equities | Level 2
Defined Benefit Plan Disclosure
Total net assets	670	262
Non-U.S. | Non-U.S. equities | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Mutual funds and other registered investment companies | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	3,210	3,286
Non-U.S. | Mutual funds and other registered investment companies | Level 1
Defined Benefit Plan Disclosure
Total net assets	3,137	3,223
Non-U.S. | Mutual funds and other registered investment companies | Level 2
Defined Benefit Plan Disclosure
Total net assets	73	63
Non-U.S. | Mutual funds and other registered investment companies | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Commingled funds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	24	23
Non-U.S. | Commingled funds | Level 1
Defined Benefit Plan Disclosure
Total net assets	24	23
Non-U.S. | Commingled funds | Level 2
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Commingled funds | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Debt securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	8,125	5,932
Non-U.S. | Debt securities | Level 1
Defined Benefit Plan Disclosure
Total net assets	6,705	4,307
Non-U.S. | Debt securities | Level 2
Defined Benefit Plan Disclosure
Total net assets	1,420	1,615
Non-U.S. | Debt securities | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	10	9
Non-U.S. | Real estate | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	4	4
Non-U.S. | Real estate | Level 1
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Real estate | Level 2
Defined Benefit Plan Disclosure
Total net assets	2	3
Non-U.S. | Real estate | Level 3
Defined Benefit Plan Disclosure
Total net assets	2	1	1
Non-U.S. | Annuity contracts | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	5	5
Non-U.S. | Annuity contracts | Level 1
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Annuity contracts | Level 2
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Annuity contracts | Level 3
Defined Benefit Plan Disclosure
Total net assets	5	5	10
Non-U.S. | Derivatives | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	1,005	1,590
Non-U.S. | Derivatives | Level 1
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Derivatives | Level 2
Defined Benefit Plan Disclosure
Total net assets	1,005	1,590
Non-U.S. | Derivatives | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Other investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	312	275
Non-U.S. | Other investments | Level 1
Defined Benefit Plan Disclosure
Total net assets	0	1
Non-U.S. | Other investments | Level 2
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Other investments | Level 3
Defined Benefit Plan Disclosure
Total net assets	312	274	210
Non-U.S. | Cash and short-term investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	132	89
Non-U.S. | Cash and short-term investments | Level 1
Defined Benefit Plan Disclosure
Total net assets	129	86
Non-U.S. | Cash and short-term investments | Level 2
Defined Benefit Plan Disclosure
Total net assets	3	3
Non-U.S. | Cash and short-term investments | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Other investment liabilities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure
Total net assets	(4,650)	(2,941)
Non-U.S. | Other investment liabilities | Level 1
Defined Benefit Plan Disclosure
Total net assets	0	(3)
Non-U.S. | Other investment liabilities | Level 2
Defined Benefit Plan Disclosure
Total net assets	(4,650)	(2,938)
Non-U.S. | Other investment liabilities | Level 3
Defined Benefit Plan Disclosure
Total net assets	0	0
Non-U.S. | Securities | Net Asset Value
Defined Benefit Plan Disclosure
Total net assets	14	15
Pension Plans | U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 13,309	$ 12,717	11,490
Allocable interest (as a percent)	98.00%	98.00%
Pension Plans | Non-U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 7,831	$ 7,556	6,699
Postretirement Benefit Plans | U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 331	$ 345	345
Allocable interest (as a percent)	2.00%	2.00%
Postretirement Benefit Plans | Non-U.S.
Defined Benefit Plan Disclosure
Total net assets	$ 1,146	$ 1,127	$ 1,036