Postretirement Benefits Other Than Pensions - Schedule of Weighted Average Assumed Annual Rate of Increase in Cost of Health Care Benefits (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Increase on service and interest cost components	$ 217
Increase on projected benefit obligations	3,268
Decrease on service and interest cost components	173
Decrease on projected benefit obligations	$ 2,647