LOANS - Consumer Troubled Debt Restructurings (Details) - Consumer $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) loan	Sep. 30, 2019 USD ($) loan	Sep. 30, 2020 USD ($) loan	Sep. 30, 2019 USD ($) 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
In North America offices
Loans receivable
Number of loans modified | loan	50,248	67,822	169,937	205,077
Post-modification recorded investment	$ 320.0	$ 350.0	$ 964.0	$ 1,071.0
TDR loans in payment default	$ 101.0	$ 98.0	$ 325.0	$ 300.0
In North America offices | Residential first mortgages
Loans receivable
Number of loans modified | loan	237	175	812	805
Post-modification recorded investment	$ 42.0	$ 26.0	$ 137.0	$ 120.0
Average interest rate reduction	0.00%	0.00%	0.00%	0.00%
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 2.0	$ 3.0	$ 10.0	$ 15.0
TDR loans in payment default	24.0	19.0	59.0	69.0
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	$ 1.0	$ 2.0	$ 7.0	$ 9.0
In North America offices | Home equity loans
Loans receivable
Number of loans modified | loan	62	219	227	613
Post-modification recorded investment	$ 5.0	$ 24.0	$ 22.0	$ 66.0
Average interest rate reduction	0.00%	1.00%	1.00%	1.00%
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 0.3	$ 2.0	$ 2.0	$ 6.0
TDR loans in payment default	6.0	4.0	12.0	11.0
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	$ 0.2	$ 2.0	$ 1.0	$ 5.0
In North America offices | Credit cards
Loans receivable
Number of loans modified | loan	48,909	66,925	167,082	202,453
Post-modification recorded investment	$ 261.0	$ 296.0	$ 786.0	$ 874.0
Average interest rate reduction	17.00%	17.00%	13.00%	17.00%
TDR loans in payment default	$ 70.0	$ 74.0	$ 251.0	$ 217.0
In North America offices | Personal, small business and other
Loans receivable
Number of loans modified | loan	1,040	503	1,816	1,206
Post-modification recorded investment	$ 12.0	$ 4.0	$ 19.0	$ 11.0
Average interest rate reduction	6.00%	6.00%	4.00%	5.00%
TDR loans in payment default	$ 1.0	$ 1.0	$ 3.0	$ 3.0
In offices outside North America
Loans receivable
Number of loans modified | loan	38,495	24,515	99,202	77,622
Post-modification recorded investment	$ 249.0	$ 140.0	$ 603.0	$ 424.0
TDR loans in payment default	$ 73.0	$ 54.0	$ 190.0	$ 173.0
In offices outside North America | Residential first mortgages
Loans receivable
Number of loans modified | loan	696	572	1,874	1,935
Post-modification recorded investment	$ 21.0	$ 22.0	$ 80.0	$ 59.0
Average interest rate reduction	1.00%	0.00%	5.00%	0.00%
TDR loans in payment default	$ 6.0	$ 1.0	$ 17.0	$ 8.0
In offices outside North America | Credit cards
Loans receivable
Number of loans modified | loan	25,147	16,703	65,738	53,649
Post-modification recorded investment	$ 122.0	$ 66.0	$ 289.0	$ 214.0
Average interest rate reduction	14.00%	17.00%	16.00%	16.00%
TDR loans in payment default	$ 47.0	$ 34.0	$ 118.0	$ 109.0
In offices outside North America | Personal, small business and other
Loans receivable
Number of loans modified | loan	12,652	7,240	31,590	22,038
Post-modification recorded investment	$ 106.0	$ 52.0	$ 234.0	$ 151.0
Average interest rate reduction	10.00%	8.00%	10.00%	9.00%
TDR loans in payment default	$ 20.0	$ 19.0	$ 55.0	$ 56.0
Deferred principal | In North America offices
Loans receivable
Post-modification recorded investment	0.0	1.0	0.0	2.0
Deferred principal | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Deferred principal | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0.0	1.0	0.0	2.0
Deferred principal | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Deferred principal | In North America offices | Personal, small business and other
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Deferred principal | In offices outside North America
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Deferred principal | In offices outside North America | Residential first mortgages
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Deferred principal | In offices outside North America | Credit cards
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Deferred principal | In offices outside North America | Personal, small business and other
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In North America offices
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In North America offices | Personal, small business and other
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In offices outside North America
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In offices outside North America | Residential first mortgages
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In offices outside North America | Credit cards
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Contingent principal forgiveness | In offices outside North America | Personal, small business and other
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Principal forgiveness | In North America offices
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Principal forgiveness | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Principal forgiveness | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Principal forgiveness | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Principal forgiveness | In North America offices | Personal, small business and other
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Principal forgiveness | In offices outside North America
Loans receivable
Post-modification recorded investment	4.0	4.0	13.0	13.0
Principal forgiveness | In offices outside North America | Residential first mortgages
Loans receivable
Post-modification recorded investment	0.0	0.0	0.0	0.0
Principal forgiveness | In offices outside North America | Credit cards
Loans receivable
Post-modification recorded investment	2.0	2.0	7.0	8.0
Principal forgiveness | In offices outside North America | Personal, small business and other
Loans receivable
Post-modification recorded investment	$ 2.0	$ 2.0	$ 6.0	$ 5.0