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Postretirement Benefits - Schedule of Amounts Recognized in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net amounts recognized at year end	$ (208,770)	$ (182,233)	$ (150,071)	$ (193,380)
Pension plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	200,465	174,470
Prior service cost	4,713	5,345
Net amounts recognized at year end	$ 205,178	$ 179,815