ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Nonperforming Loans and Leases by Class (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down	$ 1	$ 2	$ 3	$ 3
Nonperforming loans	793		793		$ 797
Loans accruing and 90 days or more past due	30		30		25
GNMA
Financing Receivable, Past Due [Line Items]
Nonperforming loans sold with right to repurchase	195		195		133
Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans accruing and 90 days or more past due	12		12		12
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Nonperforming loans	228		228		194
Loans accruing and 90 days or more past due	1		1		1
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Nonperforming loans	49		49		7
Loans accruing and 90 days or more past due	0		0		0
Commercial | Leases
Financing Receivable, Past Due [Line Items]
Nonperforming loans	4		4		0
Loans accruing and 90 days or more past due	1		1		0
Commercial | Commercial Banking
Financing Receivable, Past Due [Line Items]
Nonperforming loans	281		281		201
Loans accruing and 90 days or more past due	2		2		1
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Nonperforming loans	147		147		136
Loans accruing and 90 days or more past due	15		15		15
Retail | Home equity loans
Financing Receivable, Past Due [Line Items]
Nonperforming loans	36		36		50
Loans accruing and 90 days or more past due	0		0		0
Retail | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Nonperforming loans	184		184		231
Loans accruing and 90 days or more past due	0		0		0
Retail | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Nonperforming loans	14		14		17
Loans accruing and 90 days or more past due	0		0		0
Retail | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Nonperforming loans	13		13		15
Loans accruing and 90 days or more past due	0		0		0
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Nonperforming loans	69		69		81
Loans accruing and 90 days or more past due	0		0		0
Retail | Education
Financing Receivable, Past Due [Line Items]
Nonperforming loans	17		17		38
Loans accruing and 90 days or more past due	3		3		2
Retail | Credit cards
Financing Receivable, Past Due [Line Items]
Nonperforming loans	21		21		20
Loans accruing and 90 days or more past due	0		0		0
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Nonperforming loans	11		11		8
Loans accruing and 90 days or more past due	10		10		7
Retail | Retail
Financing Receivable, Past Due [Line Items]
Nonperforming loans	512		512		596
Loans accruing and 90 days or more past due	$ 28		$ 28		$ 24