Quarterly report pursuant to Section 13 or 15(d)

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

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LOANS / LEASES AND ALLOWANCE FOR CREDIT LOSSES LOANS / LEASES AND ALLOWANCE FOR CREDIT LOSSES - Certain Loans Acquired Ending and Unpaid Balances (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Purchased Credit Impaired, Ending Balance $ 75,722   $ 102,380      
Loans and Leases Receivable, Allowance 667,996 $ 672,580 638,413 $ 623,064 $ 613,719 $ 597,843
Commercial            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Loans and Leases Receivable, Allowance 474,576 $ 480,308 451,091 $ 424,507 $ 422,441 $ 398,753
Commercial | Commercial and Industrial            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Purchased Credit Impaired, Ending Balance 54,942   68,338      
Commercial | Commercial Real Estate            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Purchased Credit Impaired, Ending Balance 20,780   34,042      
FirstMerit Corporation            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Purchased Credit Impaired, Ending Balance 75,722   102,380      
Purchased Credit Impaired, Unpaid Balance 116,838   156,351      
Loans and Leases Receivable, Allowance 970   0      
FirstMerit Corporation | Commercial | Commercial and Industrial            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Purchased Credit Impaired, Ending Balance 54,942   68,338      
Purchased Credit Impaired, Unpaid Balance 81,934   100,031      
Loans and Leases Receivable, Allowance 970   0      
FirstMerit Corporation | Commercial | Commercial Real Estate            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Purchased Credit Impaired, Ending Balance 20,780   34,042      
Purchased Credit Impaired, Unpaid Balance 34,904   56,320      
Loans and Leases Receivable, Allowance $ 0   $ 0