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Key Weighted Average Assumptions Used in Determining Benefit Plan Costs and Obligations (Detail) - Benefit Plan Costs
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items]    
Discount Rate 4.00% 4.80%
Rate of Compensation Increase 3.00% 3.00%
Expected Long-term rate of return on plan assets 8.00% 8.00%
Health Care Cost Trend Rate Assumed for Next Year 6.00% 7.00%
Ultimate Health Care Cost Trend Rate 4.00% 4.00%
Year that Ultimate Health Care Cost Trend Rate is reached 2018 2018