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Employee Benefits - Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost (Detail) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Health care cost trend rate      
One percent increase $ 0.00 $ 0.00 $ 0.00
One percent decrease $ 0.00 $ 0.00 $ 0.00
Pension Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.45% 4.13% 4.97%
Expected return on plan assets 7.50% 7.50% 7.50%
Rate of compensation increase 4.06% 4.07% 4.02%
Postretirement Welfare Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.59% 3.46% 3.93%
Expected return on plan assets 1.50% 1.50% 1.50%
Prior to Age 65 [Member] | Net Periodic Benefit Cost Assumptions [Member]      
Health care cost trend rate      
Health care cost trend rate 6.50% 7.00% 7.50%
After Age 65 [Member] | Net Periodic Benefit Cost Assumptions [Member]      
Health care cost trend rate      
Health care cost trend rate 6.50% 7.00% 7.50%