SECURITIES - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,162	$ 3,380
Gross Unrealized Gains	55	84
Gross Unrealized Losses	(9)	(1)
Debt securities, available for sale	3,208	3,463
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	49	600
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Debt securities, available for sale	49	600
US Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	152	172
Gross Unrealized Gains	2	0
Gross Unrealized Losses	0	0
Debt securities, available for sale	154	172
U.S. Government-Sponsored Entities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	170	160
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(1)	0
Debt securities, available for sale	170	161
Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,286	959
Gross Unrealized Gains	26	35
Gross Unrealized Losses	(1)	0
Debt securities, available for sale	1,311	994
Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,171	1,094
Gross Unrealized Gains	18	31
Gross Unrealized Losses	(6)	(1)
Debt securities, available for sale	1,183	1,124
Commercial Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	299	361
Gross Unrealized Gains	8	17
Gross Unrealized Losses	(1)	0
Debt securities, available for sale	306	378
States of the U.S. and Political Subdivisions (municipals) [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33	32
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Debt securities, available for sale	33	32
Other Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Debt securities, available for sale	$ 2	$ 2

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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