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Estimated Future Benefit Payments (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Pension Plans  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 5,510
2019 6,054
2020 6,314
2021 6,932
2022 6,986
2023 - 2027 44,677
Other Postretirement Benefit Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
2018 2,252
2019 2,454
2020 2,635
2021 2,915
2022 3,130
2023 - 2027 19,349
Supplemental Employee Retirement Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
2018 87
2019 589
2020 580
2021 723
2022 712
2023 - 2027 $ 4,062