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Postretirement Benefit Plans (Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates) (Detail) - Defined Benefit Postretirement Health And Life Coverage [Member]
$ in Thousands
12 Months Ended
Dec. 31, 2016
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components $ 0
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation 3
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components 0
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation $ 3