POSTRETIREMENT BENEFIT PLANS - Cost of Postretirement Benefit Plans and Changes in Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Principal pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost (income) of postretirement benefit plans	$ 3,559	$ 3,929	$ 4,260
Prior service cost (credit) - current year	0	(42)	0
Actuarial loss (gain) - current year	1,124	971	(111)
Reclassifications out of AOCI
Curtailment / settlement gain (loss)	0	(353)	(45)
Dispositions	0	0	0
Amortization of net actuarial gain (loss)	(3,399)	(3,439)	(3,785)
Amortization of prior service credit (cost)	(146)	(135)	(143)
Total changes in other comprehensive income	(2,421)	(2,998)	(4,084)
Cost of postretirement benefit plans and changes in other comprehensive income	1,138	931	176
Other pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost (income) of postretirement benefit plans	30	(21)	(110)
Prior service cost (credit) - current year	27	(17)	82
Actuarial loss (gain) - current year	529	1,592	464
Reclassifications out of AOCI
Curtailment / settlement gain (loss)	(3)	(12)	(2)
Dispositions	(166)	(340)	0
Amortization of net actuarial gain (loss)	(434)	(319)	(312)
Amortization of prior service credit (cost)	(1)	(3)	9
Total changes in other comprehensive income	(48)	901	241
Cost of postretirement benefit plans and changes in other comprehensive income	(18)	880	131
Principal retiree benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost (income) of postretirement benefit plans	(118)	(149)	(79)
Prior service cost (credit) - current year	(7)	(23)	0
Actuarial loss (gain) - current year	119	240	(543)
Reclassifications out of AOCI
Curtailment / settlement gain (loss)	0	4	0
Dispositions	0	0	0
Amortization of net actuarial gain (loss)	82	118	79
Amortization of prior service credit (cost)	234	232	230
Total changes in other comprehensive income	428	571	(234)
Cost of postretirement benefit plans and changes in other comprehensive income	$ 310	$ 422	$ (313)