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Weighted Average Assumptions used in Determining Net Periodic Benefit Cost of Health and Welfare Plan (Detail) - Post-Retirement Benefits
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Jan. 31, 2014
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.80% 4.00% 4.30%
Current medical trend rate 6.50% 6.50% 7.00%
Ultimate trend rate 5.00% 5.00% 5.00%
Year when ultimate trend rate will be reached 2022 2018 2018