DERIVATIVES - Amounts Recorded in Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Carrying amount of hedged assets	$ 0	$ 0
Last-of-layer hedging amount	2,000
Carrying value of hedged asset in lay-of-layer hedging relationship	15,000
Debt securities available for sale
Derivative [Line Items]
Carrying amount of hedged assets	14,995	15,798
Cumulative amount of fair value hedging adjustments included in the carrying amount of the hedged items	100	(8)
Residential mortgages
Derivative [Line Items]
Carrying amount of hedged assets		976
Cumulative amount of fair value hedging adjustments included in the carrying amount of the hedged items		17
Debt securities available for sale
Derivative [Line Items]
Carrying amount of the hedged liabilities	0	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	4,083	4,689
Cumulative amount of fair value hedging adjustments included in the carrying amount of long-term borrowed funds	$ 142	$ 50

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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+ References

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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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Amount of asset hedged in last of layer, expected to be remaining at end of 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 4EEE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999718-113959

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Amount of liability hedged in fair value hedging relationship.

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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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