RETIREMENT BENEFITS - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Change in accumulated other comprehensive income (loss)
Balance, beginning of period	$ 201,672
Actuarial assumptions changes and plan experience	516		$ (760)
Net asset (loss) gain due to difference between actual and expected returns	(451)		625
Net amortization	58		229
Prior service cost	0		(4)
Curtailment/settlement gain	4		17
Foreign exchange impact and other	(36)		(93)
Impact of Tax Reform	0		(1,020)
Change in deferred taxes, net	(3)		(13)
Change, net of tax	88	$ (12)	(1,019)
Balance, end of period	202,866	228,997	201,672
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of period	(6,183)	(5,164)	(5,164)
Balance, end of period	$ (6,095)	$ (5,176)	$ (6,183)