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Employee Benefits - Pension and Other Post-Retirement Benefits (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Benefits      
Components of net periodic pension expense (credit):      
Interest cost $ 4,692,000 $ 5,660,000 $ 5,085,000
Amortization of past-service liability 0    
Expected return on plan assets (15,505,000) (13,933,000) (16,139,000)
Amortization of net actuarial loss 7,327,000 10,035,000 7,179,000
Net periodic pension (credit) expense (3,486,000) 1,762,000 (3,875,000)
Post-Retirement Benefits      
Components of net periodic pension expense (credit):      
Interest cost 327,000 512,000 513,000
Amortization of past-service liability (249,000) (249,000) (249,000)
Amortization of net actuarial loss 25,000 124,000 309,000
Net periodic pension (credit) expense $ 103,000 $ 387,000 $ 573,000