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Assumptions Used to Determine Net Periodic Benefit Cost (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.15% 4.40% 4.05%
Expected long-term return on plan assets 7.50% 7.50% 7.75%
Rate of compensation increase 4.00% 4.00% 4.00%
Post-Retirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.40% 4.55% 4.00%
Rate of compensation increase 4.00% 4.00% 4.00%