LOANS - Performance of Modified Consumer Loans (Details) - Consumer loans - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
In North America offices
Financing Receivable, Modified [Line Items]
Total modified loans balance	$ 1,358	$ 1,238
Gross credit losses	233	205
In North America offices | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	957	836
In North America offices | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	203	203
In North America offices | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	198	199
In offices outside North America
Financing Receivable, Modified [Line Items]
Total modified loans balance	388	404
Gross credit losses	6	5
In offices outside North America | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	375	392
In offices outside North America | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	8	8
In offices outside North America | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	5	4
Residential mortgages | In North America offices
Financing Receivable, Modified [Line Items]
Total modified loans balance	146	164
Gross credit losses	0	0
Residential mortgages | In North America offices | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	82	70
Residential mortgages | In North America offices | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	15	22
Residential mortgages | In North America offices | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	49	72
Residential mortgages | In offices outside North America
Financing Receivable, Modified [Line Items]
Total modified loans balance	328	334
Gross credit losses	0	0
Residential mortgages | In offices outside North America | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	323	331
Residential mortgages | In offices outside North America | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	4	2
Residential mortgages | In offices outside North America | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	1	1
Home equity loans | In North America offices
Financing Receivable, Modified [Line Items]
Total modified loans balance	14	21
Gross credit losses	0	0
Home equity loans | In North America offices | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	10	14
Home equity loans | In North America offices | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	1	1
Home equity loans | In North America offices | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	3	6
Credit cards | In North America offices
Financing Receivable, Modified [Line Items]
Total modified loans balance	1,180	1,039
Gross credit losses	231	204
Credit cards | In North America offices | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	849	740
Credit cards | In North America offices | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	186	179
Credit cards | In North America offices | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	145	120
Credit cards | In offices outside North America
Financing Receivable, Modified [Line Items]
Total modified loans balance	38	43
Gross credit losses	5	4
Credit cards | In offices outside North America | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	32	37
Credit cards | In offices outside North America | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	2	3
Credit cards | In offices outside North America | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	4	3
Personal, small business and other | In North America offices
Financing Receivable, Modified [Line Items]
Total modified loans balance	18	14
Gross credit losses	2	1
Personal, small business and other | In North America offices | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	16	12
Personal, small business and other | In North America offices | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	1	1
Personal, small business and other | In North America offices | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	1	1
Personal, small business and other | In offices outside North America
Financing Receivable, Modified [Line Items]
Total modified loans balance	22	27
Gross credit losses	1	1
Personal, small business and other | In offices outside North America | Current
Financing Receivable, Modified [Line Items]
Total modified loans balance	20	24
Personal, small business and other | In offices outside North America | 30–89 days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	2	3
Personal, small business and other | In offices outside North America | 90+ days past due
Financing Receivable, Modified [Line Items]
Total modified loans balance	$ 0	$ 0