DERIVATIVES - Cash Flow Hedges (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) recognized in AOCI on derivative	$ 544	$ (217)	$ 790	$ (545)
Total gain (loss) recognized in AOCI	680	(123)	1,058	(422)
Cash flow hedge gain expected to be reclassified from AOCI within 12 months			$ 91
Maximum length of time hedged in cash flow hedge			10 years
Interest rate contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) recognized in AOCI on derivative	545	(222)	$ 799	(544)
Foreign exchange contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) recognized in AOCI on derivative	(1)	5	(9)	(1)
Other
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	(2)	(6)	(4)	(4)
Other \| Interest rate contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	0	0	0	0
Other \| Foreign exchange contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	(2)	(6)	(4)	(4)
Net interest revenue
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	(134)	(88)	(264)	(119)
Net interest revenue \| Interest rate contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	(134)	(88)	(264)	(119)
Net interest revenue \| Foreign exchange contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	$ 0	$ 0	$ 0	$ 0

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

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Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount, before tax and after reclassification, of gain (loss) from derivative instrument designated and qualifying cash flow hedge included in assessment of hedge effectiveness, attributable to parent.

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Amount, before tax and reclassification, of gain (loss) from derivative instrument designated and qualifying cash flow hedge included in assessment of hedge effectiveness.

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Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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