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Postretirement Benefit Plans (Assumptions Used) (Detail)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Postretirement Health And Life Coverage [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 4.00% 4.25%  
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase 3.00% 3.25%  
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 4.25% 4.00% 4.75%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase 3.25% 3.00% 3.75%
Director's Retirement Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 4.00% 4.25%  
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Change In Consumer Price Index 2.00% 2.25%  
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 4.25% 4.00% 4.75%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Changes In Consumer Price Index 2.25% 2.25% 2.75%