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Retirement and Deferred Compensation Plans - Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Detail) - Postretirement Benefits [Member]
$ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect on service and interest cost components, 1% Increase $ 1
Effect on postretirement benefit obligation, 1% Increase 4
Effect on service and interest cost components, 1% Decrease (1)
Effect on postretirement benefit obligation, 1% Decrease $ (3)