LOANS AND LEASES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	$ 500		$ 500		$ 583
Other loans held for sale	724		724		42
Commercial Loan Syndication
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other loans held for sale	640		640
Level 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	500		500		583
Other loans held for sale	0		0		0
Level 2 | Residential loans held for sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	349		349		504
Level 2 | Commercial real estate loans held for sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	151		151		79
Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDRs sold		$ 310		$ 310
Pre-tax gain		72		72
Impaired Loans | Home equity loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDRs sold		55		55
Impaired Loans | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDRs sold		255		255
Transfer To Other Loans Held For Sale | Home equity loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other loans held for sale	29		29
Transfer To Other Loans Held For Sale | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other loans held for sale	6		6
Transfer To Other Loans Held For Sale | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other loans held for sale	49		49
Transfer To Other Loans Held For Sale | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other loans held for sale	78		78
Retail | Education
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans purchased	63	126	795	843
Retail | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans purchased		200	153	534
Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans purchased		222		405
Loans sold	0	163	206	444
Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans pledged as collateral for FHLB borrowed funds	24,200		24,200		24,000
Loans pledged as collateral to support the contingent ability to borrow at the FRB discount window	17,500		17,500		$ 16,800
Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans sold	$ 0	$ 14	$ 596	$ 132