POSTRETIREMENT BENEFIT PLANS - Cost of Principal Retiree Benefit Plans (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]
Total cost	$ 4,056	$ 4,112	$ 5,045
Principal retiree benefit plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost for benefits earned	94	123	145
Prior service credit amortization	(171)	(164)	(8)
Expected return on plan assets	(36)	(43)	(48)
Interest cost on benefit obligations	224	249	335
Net actuarial gain amortization	(80)	(50)	(25)
Curtailment loss (gain)	4	0	(225)
Total cost	$ 35	$ 115	$ 174