ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Loans with Indicators of High Credit Risk (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total	$ 114,720	$ 110,617
High loan-to-value
Financing Receivable, Recorded Investment [Line Items]
Total	620	796
High loan-to-value | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	339	366
High loan-to-value | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	107	166
High loan-to-value | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	174	264
High loan-to-value | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
High loan-to-value | Education
Financing Receivable, Recorded Investment [Line Items]
Total		0
Interest-only/negative amortization
Financing Receivable, Recorded Investment [Line Items]
Total	1,786	1,764
Interest-only/negative amortization | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	1,786	1,763
Interest-only/negative amortization | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Interest-only/negative amortization | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Interest-only/negative amortization | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Interest-only/negative amortization | Education
Financing Receivable, Recorded Investment [Line Items]
Total		1
Low introductory rate
Financing Receivable, Recorded Investment [Line Items]
Total	211	197
Low introductory rate | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Low introductory rate | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Low introductory rate | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Low introductory rate | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	211	197
Low introductory rate | Education
Financing Receivable, Recorded Investment [Line Items]
Total		0
Multiple characteristics and other
Financing Receivable, Recorded Investment [Line Items]
Total	1	1
Multiple characteristics and other | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	1	1
Multiple characteristics and other | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Multiple characteristics and other | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Multiple characteristics and other | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Multiple characteristics and other | Education
Financing Receivable, Recorded Investment [Line Items]
Total		0
Credit risk, loans with increased credit exposure
Financing Receivable, Recorded Investment [Line Items]
Total	2,618	2,758
Credit risk, loans with increased credit exposure | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	2,126	2,130
Credit risk, loans with increased credit exposure | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	107	166
Credit risk, loans with increased credit exposure | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	174	264
Credit risk, loans with increased credit exposure | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	$ 211	197
Credit risk, loans with increased credit exposure | Education
Financing Receivable, Recorded Investment [Line Items]
Total		$ 1