Quarterly report pursuant to Section 13 or 15(d)

LOANS / LEASES AND ALLOWANCE FOR CREDIT LOSSES - Narrative (Details)

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LOANS / LEASES AND ALLOWANCE FOR CREDIT LOSSES - Narrative (Details)
 6 Months Ended
Jun. 30, 2017
USD ($)
reserve
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jun. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Loans and leases receivable net premium $ 245,000,000   $ 120,000,000      
Loans held for investment 103,741,000   82,319,000      
Allowance for loan and lease losses $ (667,996,000) $ (672,580,000) (638,413,000) $ (623,064,000) $ (613,719,000) $ (597,843,000)
Number of days past due 30 days          
Allowance number of reserves | reserve 2          
Collateral for secured borrowings $ 30,400,000,000          
FirstMerit Corporation            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Allowance for loan and lease losses (970,000)   0      
Commercial            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Allowance for loan and lease losses $ (474,576,000) (480,308,000) (451,091,000) (424,507,000) (422,441,000) (398,753,000)
Threshold period past due for nonperforming status 90 days          
Threshold outstanding balance for quarterly impairment evaluation $ 1,000,000.0          
Threshold period past due for write-off 90 days          
Commercial | Commercial and Industrial | FirstMerit Corporation            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Allowance for loan and lease losses $ (970,000)   0      
Commercial | Commercial Real Estate | FirstMerit Corporation            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Allowance for loan and lease losses 0   0      
Consumer            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Allowance for loan and lease losses $ (193,420,000) $ (192,272,000) $ (187,322,000) $ (198,557,000) $ (191,278,000) $ (199,090,000)
Threshold period past due for nonperforming status 120 days          
Threshold period past due for write-off 120 days