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Postretirement Benefit Plans (Narrative) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Postretirement benefit health insurance reimbursable premium $ 5,000  
Defined Benefit Postretirement Health Insurance [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined benefit plan, minimum retirement age 55 years  
Defined Benefit Plan, Minimum Service Period Requirement 20 years  
Defined benefit plan, coverage period 5 years  
Defined Benefit Postretirement Life Insurance [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined benefit plan, minimum retirement age 55 years  
Defined Benefit Plan, Minimum Service Period Requirement 20 years  
Defined Benefit Plan, Benefit Description, Until Age 65 The insurance amount will be $50,000 until age 65.  
Life insurance coverage amount until age 65 $ 50,000,000  
Defined Benefit Plan, Benefit Description, After Age 65 After age 65, the insurance amount will decrease by $5,000 per year until age 74.  
Decrease in Coverage Amount After Age 65 $ 5,000,000  
Life insurance coverage amount 5,000,000  
Director's Retirement Plan [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Unrecognized Prior Service Cost Generated By Adoption of the Plan 274,000,000  
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Prior Service Cost, before Tax 22,000,000  
Defined Benefit Plan, Accumulated Benefit Obligation 1,116,000 $ 1,122,000
Defined Benefit Plan, Amount to be Amortized from Accumulated Other Comprehensive Income (Loss) Next Fiscal Year 22,000  
Cash surrender value of bank owned life insurance 3,905,000,000 3,834,000,000
Defined Benefit Postretirement Health And Life Coverage [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Accumulated Benefit Obligation 508,000 $ 540,000
Defined Benefit Plan, Amount to be Amortized from Accumulated Other Comprehensive Income (Loss) Next Fiscal Year $ 25,000,000