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Postretirement Benefit Plans - Defined Contribution Expenses for Company's Qualified Defined Contribution Plans and Statutory Profit Sharing Contributions (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items]      
Total defined contribution expense $ 14,142 $ 12,030 $ 11,799
Retirement Contributions [Member]      
Defined Contribution Plan Disclosure [Line Items]      
Total defined contribution expense 8,035 7,328 7,617
Profit Sharing Contributions [Member]      
Defined Contribution Plan Disclosure [Line Items]      
Total defined contribution expense $ 6,107 $ 4,702 $ 4,182