POSTRETIREMENT BENEFIT PLANS - Cost of Postretirement Benefit Plans and Changes in Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost of postretirement benefit plans	$ 4,056	$ 4,112	$ 5,045
Prior service cost (credit) – current year	(8)	(61)	(2,401)
Net actuarial loss (gain) – current year	(310)	4,038	(1,604)
Reclassification out of AOCI:
Net curtailment gain (loss)	(88)	(50)	76
Prior service credit (cost) amortization	(114)	(140)	(197)
Net actuarial gain (loss) amortization	(3,161)	(2,655)	(3,552)
Total changes in other comprehensive income	(3,681)	1,132	(7,678)
Cost of postretirement benefit plans and changes in other comprehensive income	375	5,244	(2,633)
Principal pension plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost of postretirement benefit plans	3,687	3,623	4,498
Prior service cost (credit) – current year	0	0	902
Net actuarial loss (gain) – current year	474	2,317	(1,022)
Reclassification out of AOCI:
Net curtailment gain (loss)	(64)	(31)	(105)
Prior service credit (cost) amortization	(290)	(303)	(205)
Net actuarial gain (loss) amortization	(2,812)	(2,449)	(3,288)
Total changes in other comprehensive income	(2,692)	(466)	(3,718)
Cost of postretirement benefit plans and changes in other comprehensive income	995	3,157	780
Other pension plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost of postretirement benefit plans	334	374	373
Prior service cost (credit) – current year	0	(54)	(12)
Net actuarial loss (gain) – current year	(656)	1,989	(164)
Reclassification out of AOCI:
Net curtailment gain (loss)	(20)	(19)	6
Prior service credit (cost) amortization	5	(1)	0
Net actuarial gain (loss) amortization	(429)	(256)	(289)
Total changes in other comprehensive income	(1,100)	1,659	(459)
Cost of postretirement benefit plans and changes in other comprehensive income	(766)	2,033	(86)
Principal retiree benefit plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost of postretirement benefit plans	35	115	174
Prior service cost (credit) – current year	(8)	(7)	(3,291)
Net actuarial loss (gain) – current year	(128)	(268)	(418)
Reclassification out of AOCI:
Net curtailment gain (loss)	(4)	0	175
Prior service credit (cost) amortization	171	164	8
Net actuarial gain (loss) amortization	80	50	25
Total changes in other comprehensive income	111	(61)	(3,501)
Cost of postretirement benefit plans and changes in other comprehensive income	$ 146	$ 54	$ (3,327)