Quarterly report pursuant to Section 13 or 15(d)

COMMITMENTS AND CONTINGENT LIABILITIES - Narrative (Details)

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COMMITMENTS AND CONTINGENT LIABILITIES - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Sep. 30, 2015
Jun. 30, 2017
Dec. 31, 2016
Sep. 28, 2015
Loss Contingencies [Line Items]          
Maturity period of majority of standby letters of credit     P2Y    
Maturity period of Commercial letters of credit     P90D    
Maturity period of forward contracts relating mortgage banking business     1 year    
Cyberco Litigation          
Loss Contingencies [Line Items]          
Bankruptcy Court recommended judgment amount in Cyberco case, principal         $ 72,000,000
Loss contingency period increase (decrease) $ (42,000,000) $ 38,000,000      
Standby Letters of Credit          
Loss Contingencies [Line Items]          
Carrying amount of deferred revenue associated with guarantees 8,000,000   $ 8,000,000 $ 8,000,000  
Outstanding standby letters of credit 559,903,000   559,903,000 637,182,000  
Commercial letters-of-credit          
Loss Contingencies [Line Items]          
Outstanding standby letters of credit 11,952,000   11,952,000 4,610,000  
Commitments to Sell Loans          
Loss Contingencies [Line Items]          
Commitments to sell residential real estate loans 1,100,000,000   1,100,000,000 $ 819,000,000  
Maximum          
Loss Contingencies [Line Items]          
Aggregate range of reasonably possible losses current legal proceedings 70,000,000   70,000,000    
Minimum          
Loss Contingencies [Line Items]          
Aggregate range of reasonably possible losses current legal proceedings 0   $ 0    
Cash [Member] | FirstMerit Overdraft Litigation [Member]          
Loss Contingencies [Line Items]          
Litigation Settlement, Amount 9,000,000        
Debt forgiveness [Member] | FirstMerit Overdraft Litigation [Member]          
Loss Contingencies [Line Items]          
Litigation Settlement, Amount 7,000,000        
Insurance Settlement [Member] | FirstMerit Overdraft Litigation [Member]          
Loss Contingencies [Line Items]          
Litigation Settlement, Amount $ 4,400,000