FAIR VALUE - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	$ 24,915	$ 19,936
Marketable equity securities	79	84
Total derivative assets	659	640
Total derivative liabilities	654	636
U.S. Treasury
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	11,836	10,508
Residential mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	10,110	6,686
Commercial mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	2,361	2,131
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	14	12
Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	24,915	19,936
Marketable equity securities	79	84
Loans held for sale	50	38
Total derivative assets	659	640
Total derivative liabilities	654	636
Fair Value, Recurring | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	659	640
Total derivative liabilities	654	636
Fair Value, Recurring | Interest rate contracts | Qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Interest rate contracts | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	573	530
Total derivative liabilities	590	518
Fair Value, Recurring | Foreign exchange contract | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	78	104
Total derivative liabilities	63	117
Fair Value, Recurring | Other derivative | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	8	6
Total derivative liabilities	1	1
Fair Value, Recurring | Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Marketable equity securities	32	36
Loans held for sale	0	0
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 1 | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 1 | Interest rate contracts | Qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 1 | Interest rate contracts | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 1 | Foreign exchange contract | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 1 | Other derivative | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	24,754	19,779
Marketable equity securities	47	48
Loans held for sale	50	38
Total derivative assets	650	633
Total derivative liabilities	653	635
Fair Value, Recurring | Level 2 | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	650	633
Total derivative liabilities	653	635
Fair Value, Recurring | Level 2 | Interest rate contracts | Qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 2 | Interest rate contracts | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	572	529
Total derivative liabilities	590	518
Fair Value, Recurring | Level 2 | Foreign exchange contract | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	78	104
Total derivative liabilities	63	117
Fair Value, Recurring | Level 2 | Other derivative | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	161	157
Marketable equity securities	0	0
Loans held for sale	0	0
Total derivative assets	9	7
Total derivative liabilities	1	1
Fair Value, Recurring | Level 3 | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	9	7
Total derivative liabilities	1	1
Fair Value, Recurring | Level 3 | Interest rate contracts | Qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 3 | Interest rate contracts | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	1	1
Total derivative liabilities	0	0
Fair Value, Recurring | Level 3 | Foreign exchange contract | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	0	0
Total derivative liabilities	0	0
Fair Value, Recurring | Level 3 | Other derivative | Non-qualifying Hedges
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total derivative assets	8	6
Total derivative liabilities	1	1
Fair Value, Recurring | U.S. Treasury
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	11,836	10,508
Fair Value, Recurring | U.S. Treasury | Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | U.S. Treasury | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	11,836	10,508
Fair Value, Recurring | U.S. Treasury | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Government agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	107	117
Fair Value, Recurring | Government agency | Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Government agency | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	107	117
Fair Value, Recurring | Government agency | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Residential mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	10,110	6,686
Fair Value, Recurring | Residential mortgage-backed securities | Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Residential mortgage-backed securities | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	10,110	6,686
Fair Value, Recurring | Residential mortgage-backed securities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Commercial mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	2,361	2,131
Fair Value, Recurring | Commercial mortgage-backed securities | Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Commercial mortgage-backed securities | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	2,361	2,131
Fair Value, Recurring | Commercial mortgage-backed securities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Corporate Bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	487	482
Fair Value, Recurring | Corporate Bonds | Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Corporate Bonds | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	326	325
Fair Value, Recurring | Corporate Bonds | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	161	157
Fair Value, Recurring | Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	14	12
Fair Value, Recurring | Municipal bonds | Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	0	0
Fair Value, Recurring | Municipal bonds | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	14	12
Fair Value, Recurring | Municipal bonds | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale	$ 0	$ 0