Annual report pursuant to Section 13 and 15(d)

LONG-TERM DEBT - Narrative (Details)

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LONG-TERM DEBT - Narrative (Details) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Feb. 28, 2015
Apr. 30, 2014
Dec. 31, 2017
Dec. 31, 2017
Aug. 07, 2017
Mar. 07, 2017
Dec. 31, 2016
Aug. 16, 2016
Aug. 09, 2016
Mar. 14, 2016
Debt Instrument [Line Items]                    
Long-term debt     $ 9,206,000,000 $ 9,206,000,000     $ 8,309,000,000      
Senior Notes | Senior notes due 2020                    
Debt Instrument [Line Items]                    
Debt face amount           $ 700,000,000.0        
Debt percent of value issued           99.994%        
Stated rate           2.375%        
Redemption price percentage     100.00%              
Senior Notes | 2.23% Huntington National Bank senior note due 2017                    
Debt Instrument [Line Items]                    
Debt face amount           $ 300,000,000.0        
Debt percent of value issued           100.00%        
Debt Instrument, Description of Variable Rate Basis     three month LIBOR              
Basis spread on variable rate     5100.00%              
Senior Notes | Huntington National Bank Senior Note Due August 2022, 2.5 Percent [Member]                    
Debt Instrument [Line Items]                    
Debt face amount         $ 700,000,000.0          
Debt percent of value issued         99.762%          
Stated rate         2.50%          
Senior Notes | Senior Notes Due January 2022                    
Debt Instrument [Line Items]                    
Debt face amount                 $ 1,000,000,000.0  
Debt percent of value issued                 99.849%  
Stated rate                 2.30%  
Debt Issuance Costs, Net                 $ 5,000,000  
Senior Notes | Three point One Five Percent Senior notes Due March 2021                    
Debt Instrument [Line Items]                    
Debt face amount                   $ 1,000,000,000.0
Debt percent of value issued                   99.803%
Stated rate                   3.15%
Debt Issuance Costs, Net                   $ 5,000,000
Subordinated Notes                    
Debt Instrument [Line Items]                    
Long-term debt     $ 5,733,000,000 5,733,000,000     4,821,000,000      
Other borrowings                    
Debt Instrument [Line Items]                    
Long-term debt     338,000,000 338,000,000     335,000,000      
Parent Company                    
Debt Instrument [Line Items]                    
Long-term debt     $ 3,128,000,000 $ 3,128,000,000     3,145,000,000      
Parent Company | Subordinated Notes | Huntington Capital II Trust Preferred 1.14% junior subordinated debentures due 2028                    
Debt Instrument [Line Items]                    
Effective rate     1.76% 1.76%            
Long-term debt     $ 31,000,000 $ 31,000,000     $ 32,000,000      
Parent Company | Subordinated Notes | Huntington Capital II Trust Preferred 1.14% junior subordinated debentures due 2028 | London Interbank Offered Rate (LIBOR) [Member]                    
Debt Instrument [Line Items]                    
Basis spread on variable rate       0.625%            
Bank | Senior Notes | Senior Notes Due February 2018 | One month prior to maturity date                    
Debt Instrument [Line Items]                    
Redemption price percentage 100.00%                  
Bank | Senior Notes | Senior Notes Due April 24, 2017, Variable Rate | One month prior to maturity date                    
Debt Instrument [Line Items]                    
Redemption price percentage   100.00%                
FirstMerit Bank | Subordinated Notes                    
Debt Instrument [Line Items]                    
Long-term Debt               $ 520,000,000