Annual report pursuant to Section 13 and 15(d)

BENEFIT PLANS - Narrative (Details)

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BENEFIT PLANS - Narrative (Details)
 1 Months Ended 12 Months Ended
Jan. 01, 2015
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
age
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Aug. 16, 2016
USD ($)
Defined Benefit Plan Disclosure [Line Items]            
Defined Benefit Plan, Fair Value of Plan Assets   $ 903,000,000 $ 903,000,000 $ 841,000,000    
Employer Contributions     0      
Amount to be Amortized from Accumulated Other Comprehensive Income (Loss) Next Fiscal Year:            
Future amortization of transition obligation (asset)   0 0      
Future amortization of prior service cost (credit)   2,000,000 2,000,000      
Future amortization gain (loss)   8,000,000 $ 8,000,000      
Defined Contribution Plan:            
Employer matching contribution, percent of match     4.00%      
Discretionary profit-share contribution, percent of annual base pay     1.00% 1.00%    
Pension Benefits            
Defined Benefit Plan Disclosure [Line Items]            
Defined Benefit Plan, Fair Value of Plan Assets   903,000,000 $ 903,000,000 $ 841,000,000 $ 874,000,000  
Employer Contributions     0 150,000,000    
Accumulated benefit obligation   900,000,000 900,000,000 851,000,000    
Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets:            
Accumulated benefit obligations in excess of plan assets, aggregate projected benefit obligation   $ 3,000,000 3,000,000      
Defined benefit plan expenses     2,000,000 2,000,000 4,000,000  
Estimated net periodic pension cost in next fiscal year     $ 9,000,000      
Defined Benefit Plan, Information about Plan Assets:            
Average duration of plan assets investment in bonds, years     13 years 8 months      
Estimated life of benefit obligations     13 years 6 months      
Pension Benefits | Equity Securities [Member]            
Defined Benefit Plan, Information about Plan Assets:            
Actual plan asset allocations   38.00% 38.00%      
Defined Benefit Plan, Target Allocation Percentage     .50      
Pension Benefits | Bonds            
Defined Benefit Plan, Information about Plan Assets:            
Actual plan asset allocations   59.00% 59.00%      
Pension Benefits | Cash and cash equivalents            
Defined Benefit Plan, Information about Plan Assets:            
Actual plan asset allocations   3.00% 3.00%      
Post-Retirement Benefits            
Defined Benefit Plan Disclosure [Line Items]            
Healthcare and life insurance benefits, employee retirement age | age     55      
Healthcare and life insurance benefits, requisite service period     10 years      
Life insurance coverage, maximum     $ 50,000      
Reduction in liability due to plan amendment $ 5,000,000          
Life insurance buyout   $ 4,000,000        
Defined Benefit Plan, Information about Plan Assets:            
Estimated future employer contributions in next fiscal year   $ 0 $ 0      
Defined Benefit Plan, Assumed Health Care Cost Trend Rates:            
Health care cost trend rate assumed for next fiscal year   6.60% 6.60%      
Ultimate health care cost trend rate   4.50% 4.50%      
Year that rate reaches ultimate trend rate     2028      
Supplemental Executive Retirement Plan            
Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets:            
Defined benefit plan expenses     $ 1,000,000 1,000,000 $ 1,000,000  
Defined Benefit Plan, Assumed Health Care Cost Trend Rates:            
Accrued pension liability   $ 35,000,000 $ 35,000,000 $ 33,000,000    
FirstMerit Bank            
Defined Benefit Plan Disclosure [Line Items]            
Defined Benefit Plan, Fair Value of Plan Assets           $ 280,000,000
Minimum | Pension Benefits | Equity Securities [Member]            
Defined Benefit Plan, Information about Plan Assets:            
Defined Benefit Plan, Target Allocation Percentage     .20      
Minimum | Pension Benefits | Bonds            
Defined Benefit Plan, Information about Plan Assets:            
Defined Benefit Plan, Target Allocation Percentage     .5      
Maximum | Pension Benefits | Bonds            
Defined Benefit Plan, Information about Plan Assets:            
Defined Benefit Plan, Target Allocation Percentage     .80