Annual report pursuant to Section 13 and 15(d)

LOANS AND LEASES AND ALLOWANCE FOR CREDIT LOSSES - Certain Loans Acquired Ending and Unpaid Balances (Details)

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LOANS AND LEASES AND ALLOWANCE FOR CREDIT LOSSES - Certain Loans Acquired Ending and Unpaid Balances (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount $ 41 $ 102
FirstMerit Bank    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 41 102
Acquired loans, unpaid balance 76 156
Commercial and Industrial | Commercial Portfolio Segment [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 39 68
Commercial and Industrial | Commercial Portfolio Segment [Member] | FirstMerit Bank    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 39 68
Acquired loans, unpaid balance 61 100
Commercial Real Estate Loan [Member] | Commercial Portfolio Segment [Member] | FirstMerit Bank    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 2 34
Acquired loans, unpaid balance 15 56
Commercial Real Estate | Commercial Portfolio Segment [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 2 34
Automobile | Consumer    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 0 0
Home Equity | Consumer    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 0 0
Residential Mortgage | Consumer    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 0 0
RV and marine finance | Consumer    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount 0 0
Consumer | Consumer    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Acquired loans, carrying amount $ 0 $ 0