FAIR VALUES OF ASSETS AND LIABILITIES - Level 3 Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024	Dec. 31, 2023
Level 3 Assets Roll Forward:
Transfers into Level 3			$ 0
Transfers out of Level 3			(31)
Derivative instruments
Opening balance	$ 3	$ (9)	3	$ (9)	$ 7	$ 2	$ 1	$ (2)
Transfers into Level 3	0	0		0
Transfers out of Level 3	(14)	(8)		(19)
Included in OCI	0	0	0	0
Purchases/originations	0	0	0	0
Repayments	0	0	0	0
Settlements	2	4	8	15
Closing balance	3	(9)	3	(9)
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date	(1)	2	4	2
Mortgage banking income
Derivative instruments
Included in earnings:	12	10	34	21
Other noninterest income
Derivative instruments
Included in earnings:	(4)	(16)	(10)	(24)
Provision for credit losses
Derivative instruments
Included in earnings:				0
Interest and fee income
Derivative instruments
Included in earnings:			0	0
MSRs
Level 3 Assets Roll Forward:
Opening balance	567	543	573	515
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains (losses) for the period:
Included in OCI	0	0	0	0
Purchases/originations	27	12	67	33
Repayments	0	0	0	0
Settlements	(17)	(15)	(48)	(40)
Closing balance	576	515	576	515	567	573	543	515
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date	(1)	(25)	(16)	7
MSRs | Mortgage banking income
Total gains (losses) for the period:
Included in earnings:	(1)	(25)	(16)	7
MSRs | Other noninterest income
Total gains (losses) for the period:
Included in earnings:	0	0	0	0
MSRs | Provision for credit losses
Total gains (losses) for the period:
Included in earnings:				0
MSRs | Interest and fee income
Total gains (losses) for the period:
Included in earnings:			0	0
Municipal securities
Level 3 Assets Roll Forward:
Opening balance	4,067	3,341	3,954	3,335
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains (losses) for the period:
Included in OCI	35	70	52	56
Purchases/originations	160	390	799	690
Repayments	0	0	0	0
Settlements	(172)	(190)	(714)	(467)
Closing balance	4,090	3,611	4,090	3,611	4,067	3,954	3,341	3,335
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date	34	68	48	52
Municipal securities | Mortgage banking income
Total gains (losses) for the period:
Included in earnings:	0	0	0	0
Municipal securities | Other noninterest income
Total gains (losses) for the period:
Included in earnings:	0	0	0	0
Municipal securities | Provision for credit losses
Total gains (losses) for the period:
Included in earnings:				(2)
Municipal securities | Interest and fee income
Total gains (losses) for the period:
Included in earnings:			(1)	(1)
Private-label CMO
Level 3 Assets Roll Forward:
Opening balance	21	20	21	20
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains (losses) for the period:
Included in OCI	0	0	0	0
Purchases/originations	0	0	0
Repayments	0	0	0	0
Settlements	2	2	2	3
Closing balance	23	22	23	22	21	21	20	20
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date	0	0	0	0
Private-label CMO | Mortgage banking income
Total gains (losses) for the period:
Included in earnings:	0	0	0	0
Private-label CMO | Other noninterest income
Total gains (losses) for the period:
Included in earnings:	0	0	0	0
Private-label CMO | Provision for credit losses
Total gains (losses) for the period:
Included in earnings:				0
Private-label CMO | Interest and fee income
Total gains (losses) for the period:
Included in earnings:			0	(1)
Asset-backed securities
Level 3 Assets Roll Forward:
Opening balance	38	35	49	75
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains (losses) for the period:
Included in OCI	0	0	0	0
Purchases/originations	0	0	0	0
Repayments	0	0	0	0
Settlements	0	(1)	(11)	(41)
Closing balance	38	34	38	34	38	49	35	75
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date	0	0	0	0
Asset-backed securities | Mortgage banking income
Total gains (losses) for the period:
Included in earnings:	0	0	0	0
Asset-backed securities | Other noninterest income
Total gains (losses) for the period:
Included in earnings:	0	0	0	0
Asset-backed securities | Provision for credit losses
Total gains (losses) for the period:
Included in earnings:				0
Asset-backed securities | Interest and fee income
Total gains (losses) for the period:
Included in earnings:			0	0
Loans held for investment
Level 3 Assets Roll Forward:
Opening balance	62	60	61	54
Transfers into Level 3	3	3	7	11
Transfers out of Level 3	0	0	0	0
Total gains (losses) for the period:
Included in OCI	0	0	0	0
Purchases/originations	0	0	0	0
Repayments	(2)	(3)	(5)	(4)
Settlements	0	0	0	0
Closing balance	63	60	63	60	$ 62	$ 61	$ 60	$ 54
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date	0	0	0	0
Loans held for investment | Mortgage banking income
Total gains (losses) for the period:
Included in earnings:	0	0	0	0
Loans held for investment | Other noninterest income
Total gains (losses) for the period:
Included in earnings:	$ 0	$ 0	0	0
Loans held for investment | Provision for credit losses
Total gains (losses) for the period:
Included in earnings:				0
Loans held for investment | Interest and fee income
Total gains (losses) for the period:
Included in earnings:			$ 0	$ (1)