DERIVATIVES - Amounts Recorded in Balance Sheet (Details) $ in Millions	Sep. 30, 2019 USD ($)
Derivative [Line Items]
Carrying amount of the hedged assets	$ 0
Cumulative hedging adjustments on discontinued fair value hedge	(1)
Debt securities available for sale
Derivative [Line Items]
Carrying amount of the hedged assets	19,872
Cumulative amount of fair value hedging adjustments included in the carrying amount of the hedged items	13
Last-of-layer hedging amount	2,000
Carrying value of hedged asset in lay-of-layer hedging relationship	20,000
Residential mortgages
Derivative [Line Items]
Carrying amount of the hedged assets	985
Cumulative amount of fair value hedging adjustments included in the carrying amount of the hedged items	26
Debt securities available for sale
Derivative [Line Items]
Carrying amount of the hedged liabilities	0
Residential mortgages
Derivative [Line Items]
Carrying amount of the hedged liabilities	0
Long-term borrowed funds
Derivative [Line Items]
Carrying amount of the hedged liabilities	5,450
Cumulative amount of fair value hedging adjustments included in the carrying amount of long-term borrowed funds	$ 62

Amortized cost of closed portfolio of prepayable financial asset and beneficial interest secured by prepayable financial instrument, used to designate hedging relationship in which hedged item is last layer expected to be remaining at end of hedging relationship.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EEE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999718-113959

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Amount of asset hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959

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Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EEE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999718-113959

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Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk, remaining after discontinued hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959

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Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959

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