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Key Assumptions Used in Determining Benefit Plan Costs and Obligations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Benefit Plan Costs      
Defined Benefit Plan Disclosure [Line Items]      
Discount Rate [1] 4.10% 4.30% 4.00%
Rate of Compensation Increase 3.00% 3.00% 3.00%
Expected Long-term rate of return on plan assets 7.75% 8.00% 8.00%
Health Care Cost Trend Rate Assumed for Next Year 8.00% 7.00% 7.00%
Ultimate Health Care Cost Trend Rate 4.00% 4.00% 4.00%
Year that Ultimate Health Care Cost Trend Rate is reached 2025 2022 2018
Effect of 1% Increase in Health Care Cost Trend Rate $ 1,625 $ 1,352 $ 1,383
Effect of 1% Decrease in Health Care Cost Trend Rate $ (1,238) $ (1,032) $ (1,040)
Benefit Obligation      
Defined Benefit Plan Disclosure [Line Items]      
Discount Rate 3.60% 4.10% 4.30%
Rate of Compensation Increase 3.00% 3.00% 3.00%
Health Care Cost Trend Rate Assumed for Next Year 7.50% 8.00% 7.00%
Ultimate Health Care Cost Trend Rate 4.50% 4.00% 4.00%
Year that Ultimate Health Care Cost Trend Rate is reached 2024 2025 2022
Effect of 1% Increase in Health Care Cost Trend Rate $ 19,629 $ 19,471 $ 14,877
Effect of 1% Decrease in Health Care Cost Trend Rate $ (15,179) $ (15,153) $ (11,611)
[1] As a result of the addition of a plan participant to the SERP in July 2015, the Company was required to update the discount rate used in determining SERP costs for the remainder of 2015. Based on an assessment of current market conditions using high quality corporate bond interest rate indices and pension yield curves at that time, the Company assumed a discount rate of 4.30% for the SERP from July through December of 2015.