LOANS - Consumer Troubled Debt Restructurings (Details) - Consumer $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) loan	Mar. 31, 2019 USD ($) loan	Dec. 31, 2019
Loans receivable
Period within which default occurred post-modification	1 year
Number of days past due, non-accrual status	60 days
Residential first mortgages
Loans receivable
Number of days past due, non-accrual status	90 days		90 days
Commercial banking
Loans receivable
Number of days past due, non-accrual status	90 days
In North America offices
Loans receivable
Number of loans modified | loan	68,074	73,302
Post-modification recorded investment	$ 361	$ 403
TDR loans in payment default	$ 107	$ 97
In North America offices | Residential first mortgages
Loans receivable
Number of loans modified | loan	277	493
Post-modification recorded investment	$ 44	$ 74
Average interest rate reduction	0.00%	0.00%
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 4	$ 7
TDR loans in payment default	14	23
In North America offices | Residential first mortgages | New OCC guidance
Loans receivable
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 3	$ 4
In North America offices | Home equity loans
Loans receivable
Number of loans modified | loan	82	206
Post-modification recorded investment	$ 8	$ 21
Average interest rate reduction	2.00%	2.00%
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 1	$ 2
TDR loans in payment default	2	3
In North America offices | Home equity loans | New OCC guidance
Loans receivable
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 1	$ 2
In North America offices | Credit cards
Loans receivable
Number of loans modified | loan	67,282	72,247
Post-modification recorded investment	$ 305	$ 305
Average interest rate reduction	17.00%	18.00%
TDR loans in payment default	$ 90	$ 70
In North America offices | Installment and other revolving
Loans receivable
Number of loans modified | loan	433	356
Post-modification recorded investment	$ 4	$ 3
Average interest rate reduction	6.00%	5.00%
TDR loans in payment default	$ 1	$ 1
In offices outside North America
Loans receivable
Number of loans modified | loan	27,505	26,862
Post-modification recorded investment	$ 139	$ 146
TDR loans in payment default	$ 56	$ 59
In offices outside North America | Residential first mortgages
Loans receivable
Number of loans modified | loan	536	725
Post-modification recorded investment	$ 14	$ 20
Average interest rate reduction	5.00%	0.00%
TDR loans in payment default	$ 6	$ 3
In offices outside North America | Credit cards
Loans receivable
Number of loans modified | loan	19,315	18,493
Post-modification recorded investment	$ 73	$ 75
Average interest rate reduction	16.00%	16.00%
TDR loans in payment default	$ 33	$ 38
In offices outside North America | Installment and other revolving
Loans receivable
Number of loans modified | loan	7,654	7,644
Post-modification recorded investment	$ 52	$ 51
Average interest rate reduction	11.00%	9.00%
TDR loans in payment default	$ 17	$ 18
Deferred principal | In North America offices
Loans receivable
Post-modification recorded investment	0	1
Deferred principal | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0	1
Deferred principal | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside North America
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside North America | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside North America | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside North America | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside North America
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside North America | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside North America | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside North America | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In offices outside North America
Loans receivable
Post-modification recorded investment	5	5
Principal forgiveness | In offices outside North America | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In offices outside North America | Credit cards
Loans receivable
Post-modification recorded investment	3	3
Principal forgiveness | In offices outside North America | Installment and other revolving
Loans receivable
Post-modification recorded investment	$ 2	$ 2