CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Change in Each Component of AOCI (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		$ 197,074
Balance, end of period		197,015	$ 202,866
Net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(2,250)	(1,158)
Adjustment to opening balance, net of taxes				$ 0	$ (3)
Adjusted balance, beginning of period				(2,250)	(1,161)
Other comprehensive income before reclassifications		1,226	(949)
Increase (decrease) due to amounts reclassified from AOCI		(91)	(109)
Total other comprehensive income		1,135	(1,058)
Balance, end of period		(1,115)	(2,219)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		192	(921)
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				192	(921)
Other comprehensive income before reclassifications		(575)	101
Increase (decrease) due to amounts reclassified from AOCI		4	27
Total other comprehensive income		(571)	128
Balance, end of period		(379)	(793)
Cash flow hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(728)	(698)
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				(728)	(698)
Other comprehensive income before reclassifications		186	(243)
Increase (decrease) due to amounts reclassified from AOCI		100	21
Total other comprehensive income		286	(222)
Balance, end of period		(442)	(920)
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(6,257)	(6,183)
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				(6,257)	(6,183)
Other comprehensive income before reclassifications		(110)	41
Increase (decrease) due to amounts reclassified from AOCI		46	47
Total other comprehensive income		(64)	88
Balance, end of period		(6,321)	(6,095)
Foreign currency translation adjustment (CTA), net of hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(28,070)	(25,708)
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				(28,070)	(25,708)
Other comprehensive income before reclassifications		58	1,120
Increase (decrease) due to amounts reclassified from AOCI		0	0
Total other comprehensive income		58	1,120
Balance, end of period		(28,012)	(24,588)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(57)	0
Adjustment to opening balance, net of taxes					0
Adjusted balance, beginning of period				(57)	0
Other comprehensive income before reclassifications		18	(4)
Increase (decrease) due to amounts reclassified from AOCI		0	0
Total other comprehensive income		18	(4)
Balance, end of period		(39)	(4)
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(37,170)	(34,668)
Adjustment to opening balance, net of taxes	[1]			0	(3)
Adjusted balance, beginning of period				$ (37,170)	$ (34,671)
Other comprehensive income before reclassifications		803	66
Increase (decrease) due to amounts reclassified from AOCI		59	(14)
Total other comprehensive income		862	52
Balance, end of period		$ (36,308)	$ (34,619)

[1] See Note 1 to the Consolidated Financial Statements for additional details.