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Postretirement Benefits - Schedule of Amounts Recognized in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net amounts recognized at year end	$ (150,071)	$ (193,380)	$ (173,883)	$ (152,149)
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	143,453	186,563
Prior service cost	5,976	4,135
Net amounts recognized at year end	$ 149,429	$ 190,698