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Benefit Plans - Actuarial Assumptions Used (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.50%    
Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.50% 4.25% 4.50%
Rate of compensation increase 0.00% 0.00% 0.00%
Expected return on plan assets 6.00% 6.00% 6.00%
Medical and life insurance benefits cost rate of increase 0.00% 0.00% 0.00%
Post-retirement [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.50% 4.25% 4.50%
Rate of compensation increase 0.00% 0.00% 0.00%
Expected return on plan assets 0.00% 0.00% 0.00%
Medical and life insurance benefits cost rate of increase 6.00% 6.00% 6.00%